mammalian sperm motility

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from meiotic division to sperm maturation. In eukar-

yotic cells, one of the most common mechanisms for

regulating protein activity is the addition and=or

removal of phosphate groups from serine, threonine

or tyrosine residues of protein moieties. Addition or

removal of phosphate can induce allosteric

modifications resulting in conformational changes

in proteins leading either to their activation or

inactivation. These processes are regulated by bothprotein kinases and protein phosphatases. In con-

trast to protein kinases that add a phosphate group

to the hydroxyl group of serine, threonine or tyro-

sine residues, phosphatases remove it. In mam-

malian spermatozoa, the ability to actively swim is

acquired during the transit through the epididymis

under the control of different factors, such as cAMP,

intracellular pH, intracellular calcium and phosphor-

ylation of sperm proteins. As the acquisition of func-

tional competence including gaining motility during

epididymal transit occurs in the complete absence ofcontemporaneous gene transcription and translation

on the part of the spermatozoa, it is widely

accepted that post-translational modifications are

the only means by which spermatozoa can become

competent.

PP1 Isoforms and their Interacting

Proteins (PIPs)

The protein serine=threonine phosphatases (PPs)

specifically hydrolyze serine=threonine phosphoe-sters, and are metalloproteins with extremely

diverse and unrelated functions [Cohen 2002]. As a

holoenzyme, the PPs have catalytic subunits and

regulatory subunits. The catalytic subunits are div-

ided into four major groups, including protein

phosphatase 1 (PP1), protein phosphatase 2A

(PP2A), protein phosphatase 2B (PP2B) and protein

phosphatase 2C (PP2C). In mammals, there are four

homologous isoforms of type 1 serine=threonine

protein phosphatase (PP1a, PP1b=d, PP1c1 and

PP1c2) [Bollen and Stalmans 1992]. These isoformsshare >89% identity and are encoded by three dis-

tinct genes, with PP1c1 and PP1c2 produced from

the alternative splicing of the same primary tran-

script. The isoforms of PP1 vary in sequence at

their extreme amino and carboxyl termini.

Functions of PP1 include controlling metabolism,

cell division, apoptosis and protein synthesis

by dephosphorylation of key regulatory proteins

[Cohen 2002; Bollen 2001; Ceulemans et al. 2002;

Ceulemans and Bollen, 2004). All PP1s contain a

Thr-Pro-Pro-Arg (TPPR) amino acid sequence

segment at their carboxyl terminal, which is a

consensus sequence for phosphorylation by

cyclin-dependent kinases (Cdks). Phosphorylation

of PP1 by Cdk1 and Cdk2 in somatic cells reduces

the catalytic activity of this enzyme [Cohen 2002;Dohadwala et al. 1994; Kwon et al. 1997; Liu et al.

1999].

PP1s do not exist freely in the cell but are

associated with a large variety of polypeptides that

determine when and where PP1 acts. These PIPs

(also called regulatory subunits) function as

activity-modulators, targeting subunits and=or sub-

strates. Hormones, growth factors and metabolites

control the function of the PP1 holoenzymes mainly

by modulating the interaction of the subunits. The

available information suggests that these PIPs inter-act with PP1 via multiple, short-sequence motifs.

The PIPs are structurally quite different, but almost

all have a consensus binding motif (RK)-x0-1-

(VI)-{P}-(FW), where x denotes any residue and {P}

any residue except Pro, and it is often called simply

the RVxF-motif. The RVxF-motif binds to a hydro-

phobic channel near the C-terminus of PP1. The

binding of the RVxF-motif not only has a PP1

anchoring function but also promotes the interaction

of secondary, lower-affinity binding sites, often

resulting in an altered activity and=or substratespecificity of PP1. The F-x-x-(RK)-x-(RK) motif

represents a new consensus sequence for the recog-

nition and binding of some Bcl-2 proteins to PP1

[Ayllon et al. 2002].

Currently, about 70 mammalian genes coding for

more than 60 PIPs have been identified [Garcia-

Gimeno et al. 2003]. Given there are approximately

10,000 phosphoproteins in mammals, many PIPs

remain to be discovered. PIPs can be classified into

mainly 8 categories of: glycogen metabolism, myofi-

briella, nuclear, endoplasmic-ribosomal, plasmamembrane=cytoskeleton centrosome=microtubule,

apoptosis and specific substrates and inhibitors. In

addition to inhibitor-1 (I1) and inhibitor-2 (I2)

representing two different ways of inhibiting PP1

phosphatase activity, there are also other protein

phosphatase inhibitors without a clear mechanism

[Garcia-Gimeno et al. 2003; Zhang et al. 1998;

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Hrabchak and Varmuza 2004; Mishra et al. 2003;

Vijayaraghavan et al. 1996; Smith et al. 1996]. I1

activity is regulated by cAMP-dependent phosphory-

lation of a single threonine residue by protein kinase

A (PKA) and by calcium=calmodulin-dependent

dephosphorylation of the same residue [Shenolikar

and Nairn 1991; Cohen 1989]. I2 binds to the catalytic

subunit of PP1 to form an inactive cytoplasmic form

of the enzyme (PP1I2) that can be converted toactive PP1 by phosphorylation of the bound I2 by

glycogen synthase kinase-3 [Bollen and Stalmans

1992; Cohen 1989; Hemmings et al. 1982]. In sperma-

tozoa of mice, a novel PP1 inhibitor 3 (I3) containing

both the RVxF-motif, nuclear localization signals

(NLS) and nuclear targeting signals (NTS) has been

discovered [Huang et al. 2005a; Han et al. 2007]. I1,

I2 and I3 are heat-stable proteins all enriched in pro-

line [Zhang et al. 1998]. It has been shown that PP1s

nuclear translocation and nuclear retention depend

on binding to RVxF-motif interactors [Lesage et al.2004]. The PP1 nuclear interactors include PNUTS,

Sds22, NIPP and SIPP1 that have both the RVxF-motif

and NLS.

Expression Localization and Possible

Functions of PP1 Isoforms

PP1 is expressed in various cellular compartments

but is most abundant in the nucleus. PP1a, PP1cand

PP1bare closely related isoforms with distinct locali-

zation patterns. In somatic cells, PP1a, PP1c andPP1b are primarily located in the nucleus. PP1a is

mainly attached to the nuclear matrix, while PP1c

is predominantly found in the nucleoli. PP1b is

present in the non-nucleolar chromatin fraction and

the nucleoli [Andreassen et al. 1998; Twinkle-Mul-

cahy et al. 2001; Twinkle-Mulcahy et al. 2003]. Using

the fluorescent fusion proteins, isoforms of PP1 have

been delicately located in mammalian cells cultured

in vitro. During interphase, PP1cwas found in both

cytoplasmic and nucleoplasmic pools, showing a

prominent accumulation within nucleoli, targetingto kinetochores and chromatin. This implicates

PP1c in multiple regulatory pathways, in agreement

with previous studies linking its activity to the

regulation of transcription, chromatin remodeling,

chromosome condensation and segregation, cytokin-

esis, and reassembly of the nuclear envelope. PP1ais

largely excluded from the nucleoli found mainly in a

diffuse pool and in a few as-yet-unidentified foci

[Twinkle-Mulcahy et al. 2006].

In addition to the nucleus, PP1 is also found

within the axoneme. PP1c is anchored in the central

pair apparatus of the axoneme in Chlamydomonas

flagellar [Yang et al. 2000]. Ciliary and flagellar

motility is controlled by phosphorylation [Brokaw

1987; Satir et al. 1993; Tash and Bracho 1994]. PP1

may be involved in the regulation of flagellar motilitytogether with protein kinases [San Agustin and

Witman 1994; Chaudhry et al. 1995]. Recently, PP1

was found to be involved in regulating the acqui-

sition of motility [Huang et al. 2004; Huang et al.

2004b; Huang et al. 2005b].

Function and Expression of PP1/PIPs

in the Testis, Epididymis and

Spermatozoa

In the Testis

PP1a, PP1band PP1care all expressed in the testis

[Tash and Bracho 1994]. Higher levels of PP1ain con-

densing spermatids and lower levels in other germ

cell stages have been found. While PP1c1 is ubiqui-

tously expressed, PP1c2 is conserved and expressed

in the testis, spermatozoa and brain [Andreassen

et al. 1998; Smith et al. 1996; Huang et al. 2002;

Kitagawa et al. 1990]. PP1c2 is expressed in the nuclei

of germ cells from the pachytene spermatocyte stage

through the early spermatid stages, and in the sper-matozoa head and flagella [Shima et al. 1993; Huang

et al. 2004a; Huang et al. 2004b; Huang et al. 2005b].

The function of PP1c has been confirmed by

PP1c= knock-out mice [Varmuza et al. 1999]. Males

homozygous for a null mutation of PP1c gene are

sterile, and display both germ cell and Sertoli cell

defects. Histopathology from PP1c mutants indi-

cates a more complex role for this protein in

spermatogenesis. More than one defect is observed

in mutant mice. The defects include elevated

serum FSH [Oppedisano-Wells et al. 2002], increasedDNA fragmentation in germ cells [Jurisicova et al.

1999] and increased aneuploidy in haploid gametes

[Oppedisano-Wells et al. 2002]. Loss of spermatids

begins at the round spermatid stage and increases

in severity such that there is a marked reduction in

elongating and condensing spermatids and an almost

complete absence of mature sperm. Meiosis may be

171 Roles of Serine/Threonine Protein Phosphatase 1


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disrupted giving rise to polyploid spermatids.

Interestingly, histones remain complexed with

spermatid chromatin beyond when they are nor-

mally removed and replaced by protamines. Normal

staging is disrupted in PP1 mutants. Some cell types

are reduced in number, but none are absolutely

missing [Varmuza et al. 1999]. A chimeric testis with

wild type of Sertoli cells and PP1c= spermatids

revealed intermediate phenotypes when comparedwith PP1c= . They did not sire pups derived from

mutant germ cells, suggesting that expression of

PP1c2 genes may be restricted to the spermatozoon

[Oppedisano-Wells and Varmuza 2003].

Spermatogenic zip protein 1 (Spz1) has been

identified as binding to the PP1c2 splice variant in

the mouse testis [Hrabchak and Varmuza 2004].

Spz1 was a member of the basic helix-loop-helix

family of transcription factors. Overexpression of

spz1 and loss of PP1 c in the testis show similar phe-

notypes such as spermatogenic arrest and germ cellapoptosis [Hsu et al. 2004].

In the Epididymis and Spermatozoa

Human and primate sperm extracts contain PP

activity that might be from PP1c2 or PP2A [Smith

et al. 1996], whereas sea urchin [Swarup and

Garbers 1992; Tash et al. 1988], caprine [Barua et al.

1985], canine and porcine [Tash et al. 1988] sperm

extracts primarily contain PP2B (Calcineurin)

activity. In bovine sperm extracts contain bothPP2B and PP1 activity [Tash et al. 1988; Hrabchak

and Varmuza 2004; Mishra et al. 2003; Huang et al.

2002; Huang et al. 2004a,b; Huang et al. 2005b;

Varmuza et al. 1999; Oppedisano-Wells and Varmuza

2003; Tang and Hoskins 1975]. In human and pri-

mate spermatozoa, the heat-stable specific inhibitor

of PP1 is neither I1 nor I2 [Smith et al. 1996]. The

I2-like inhibitor is antagonized by the addition of

Glucogen synthase kinase 3 (GSK-3) but the signifi-

cance is not clear. A new PP1 inhibitor I3 has been

identified in mouse spermatozoa [Han et al. 2007].Sds22, 14-3-3 protein and hsp90 are potential regula-

tors of PP1c2. PP1c2 may also regulate epididymal

sperm motility [Hrabchak and Varmuza 2004; Mishra

et al. 2003; Huang et al. 2004a,b; Huang et al. 2005b;

Huang et al. 2002; Shima et al. 1993; Jurisicova et al.

1999]. Phosphorylation of PP1c2 increases exhibiting

decreased activity during sperm maturation as

motility of spermatozoa increase. Three pools of

PP1c2 in caudal and caput epididymal spermatozoa

are found. The caput pool includes the active form

of PP1c2, phosphorylated and an active form of

14-3-3 binding PP1 c2 and the inactive form of

hsp90 binding PP1 c2. The cauda pool includes the

inactive form of sds22 binding PP1c2 [Mishra et al.

2003], phosphorylated and an active form of 14-3-3

binding PP1c2 and an inactive form of hsp90 bindingPP1c2 [Huang et al. 2004a,b]. PP1c2 is inactivated by

binding to sds22 and hsp90 while PP1c2 is activated

and phosphorylated by binding to 14-3-3 protein

[Huang et al. 2004a; Huang et al. 2002]. Sds22 is a

mammalian homologue of yeast PP1 binding protein

[Huang et al. 2002] belonging to a family of proteins

that contain repeats of leucine-rich 22 amino acid

segment. The 14-3-3 proteins belong to a family of

abundant and widely expressed 2833 kDa acidic

polypeptides that spontaneously self-assemble as

dimmers. The 14-3-3 proteins bind to phosphoserine=threonine containing motifs in a sequence-

specific manner [Yaffe and Elia 2001; Aitken et al.

1992; Tzivion and Avruch 2002]. The Hsp90 is a

highly conserved ATP-dependent chaperone

[Richter and Buchner 2001] and this protein is subject

to tyrosine-phosphorylation during sperm capa-

citation in mice, rats and humans (Ecroyd et al.

2003). Cytosolic PP1 and GSK-3 activities appear to

be inversely related to the motility of monkey epidi-

dymal sperm [Smith et al. 1996]. Higher concen-

tration of GSK-3 and PP1 are present in immotilebovine caput epididymal sperm compated with

motile cauda epididymal sperm, and may control

motility [Vijayaraghavan et al. 1996; Miki et al.

2004; Somanath et al. 2004].

Some Other Estimated PIPs from Database

Expressed in the Testis and/or Spermatozoa

As mentioned above, two distinct docking consen-

sus motifs (RK)-x0-1-(VI)-{P}-(FW) and F-x-x-(RK)-x-

(RK) have been identified in PIPs. The PIPs databaseat http://pp1signature.pasteur.fr/ includes all discov-

ered proteins containing the two motifs. These

include transcription factors, transport proteins as

well as kinases.

A putative transcription factor or DNA-associated

protein called SARP (several ankyrin repeat protein)

has been identified as a PIP. SARP has 3 splice

Y. Han et al. 172


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variants, SARP1, SARP2 and SARP3. SARP1 and=or

SARP2 are expressed at high levels in testis and

spermatozoa, where they are shown to interact with

both PP1c1 and PP1c2. SARP is highly abundant in

the nucleus of mammalian cells, consistent with the

putative nuclear localization signal at the N-terminus.

The presence of a lucine zipper near the C-terminus

of SARP1 and SARP2, and the binding of mammalian

DNA to SARP2, suggests that SARP1 and SARP2 maybe transcription factors or DNA-associated proteins

that modulate gene expression [Browne et al. 2007].

Angiotensin-converting enzyme (ACE) is a zinc-

containing dipeptidyl carboxypeptidase widely

distributed in mammalian tissues and is thought to

play a critical role in blood pressure regulation.

Testis contain a unique androgen-dependent ACE

isozyme, ACE-T, that is initially found in post-meiotic

step 3 spermatids and then increases markedly dur-

ing differentiation. ACE-T is strictly confined to the

adluminal membrane face of elongating spermatidsand localizing to the neck and midpiece region of

released and ejaculated spermatozoa [Pauls et al.

2003]. Male mice homozygous for a disrupted ACE

gene are almost infertile, despite showing normal

mating behavior, testis histology and sperm para-

meters Reduced oviduct transport and zona pellu-

cida binding of spermatozoa is observed [Krege

et al. 1995; Hagaman et al. 1998]. It is estimated that

the unique N-terminal of ACE-T bearing specific

binding properties for an oviduct=ovum substrate

may yield male sterility [Kessler et al. 2000].Another putative PIP is testis-specific protein

kinase (Tesk) 2 which is a member of the Tesk family

with 48% amino acid identity with Tesk1. Tesk2,

is predominantly expressed in the nucleus of

Sertoli cells. It phosphorylates cofilin=ADF (actin-

depolymerizing factor) at Ser3 that induces actin

reorganization. Since actin-depolymerizing and

actin-severing activities of cofilin=ADF are abrogated

by phosphorylation at Ser3, TESK2 seems to play an

important role in actin filament dynamics by inhibit-

ing cofilin=ADF activity [Toshima et al. 2001].Fer-1, first discovered inCaenorhabditis elegansis

another putative PIP that is mainly expressed in

spermatocytes. It is prevalent when membranous

organelles (MOs) fuse with the spermatid plasma

membrane. Resembling some viral fusion proteins,

fer-1may play a direct role in MO-plasma membrane

fusion [Achanzar and Ward 1997].

Mechanisms of Spermatozoa Flagellar

Motility Control

The force for flagellar movement is exerted

through the sliding of outer-doublet microtubules.

ATP is required to support coordinated movement

of the central axoneme and surrounding flagellar

structures [Mann and Lutwak-Mann 1981]. This is dri-

ven by dynein molecular motors [Inaba 2003] i.e., thedynein ATPase. Several studies have revealed that

ciliary and flagellar motility is controlled by phos-

phorylation [Brokaw 1987; Satir et al. 1993; Tash

and Brocho 1994; Chaudhry et al. 1995]. In vitro

analysis has indicated that cAMP-dependent protein

Skinase A, phosphatases PP1 and PP2A anchored in

the axoneme are likely involved [Yang et al. 2000; an

Agustin and Witman 1994;Chaudhry et al. 1995; Huang

et al. 1982; Porter et al. 1992; Piperno et al. 1992;

Piperno et al. 1994; Yoshimura and Shingyoji 1999].

Spermatozoa are stored in extratesticular ducts inan immotile state in many animals and their motility

is activated on their release from the ducts. Activation

of motility is inhibited by factors in the extratesticular

plasma. Mammalian spermatozoa from the distal part

of the epididymis show better motility activation than

that from the proximal part of the epididymis as they

acquire the capacity for motility and fertilization dur-

ing epididymal transit. During this passage through

the epididymis, changes are observed in the intra-

cellular second messengerscAMP, pH, calcium

and intra-sperm ATP content [Bedford and Hoskins1990; Cooper 1986]. The relation of sperm motility

activation and other morphological=physiological

change(s) of sperm during spermiogenesis are still

unclear. But, it is known that the potential for

motility already exists in both immature testicular

and epididymal sperm as evidenced by the ability

of demembranated immature sperm to undergo

motility activation [Mohri and Yanagimachi 1980;

Yeung 1984]. Sperm motility in immature spermato-

zoa can be initiated by stimulating protein kinase

activity or inhibiting protein phosphatase activity[Vijayaraghavan et al. 1996; Smith et al. 1996; Smith

et al. 1999]. So, rather than acquiring the capacity

for motility, spermatozoa in the epididymis might

make themselves more sensitive to stimulating fac-

tors by gradually expelling inhibitors or by other

unknown mechanisms. One possible signal is phos-

phorylation=dephosphorylation. Low protein kinase



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and high protein phosphatase activities most likely

limit motility in immature spermatozoa. In a number

of species, development of sperm motility in the epi-

didymis is associated with increased intrasperm

cAMP [Amann et al. 1982; Brandt and Hoskins

1980], associated with decreased glycogen synthase

kinase-3 and protein phosphatase 1 activity [Vijayar-

aghavan et al. 1996]. It is noted that the control fac-

tors of phosphatase to sperm motility might beconfined to the axoneme [Habermacher and

Sale 1996]. This is supported by the observation of

the gain of motility after the addition of protein

phosphatase 1 inhibitors to demembraned fowl

spermatozoa [Ashizawa et al. 1994].

AKAPs [A Kinase Anchor Protein, cAMP-depen-

dent protein kinase anchoring proteins, Miki and

Eddy 1998] are the candidate target of PP1 in sperm

flagella. AKAPs assemble multi-enzyme signaling

complexes in proximity to substrate effector pro-

teins, thus directing and amplifying membrane-generated signals. They form a transduceosome,

an autonomous multivalent scaffold that assembles

and integrates signals derived from multiple path-

ways. The AKAP family shares little overall primary

sequence similarity excluding their functional

domains including the targeting domain (as a scaf-

fold and membrane anchor) and the amphipathic

helical tethering domain (binding to regulatory sub-

units) that are highly conserved. In addition to bind-

ing to cAMP-dependent protein kinases such as

protein kinase A, some AKAPs associate with proteinkinase C and Ser=Thr phosphates.

It has been shown that the amino terminus of

AKAP121, a potential PIP, interacts with mitochon-

drial membranes and with tubulin. Tubulin, a hetero-

dimer composed of two similar acidic isoforms

(a and b) participates in the organization of

eukaryotic microtubule networks. AKAP=PKA or

AKAP=PP1 complexes anchored to spindles might

regulate the dynamic assembly of microtubules by

creating target sites of cAMP action [Cardone et al.

2002]. In contrast to AKAP121, the candidacy ofAKAP4 as PIP is additionally experimentally sup-

ported. AKAP4 is the major fibrous sheath protein

located in the principal piece of spermatozoa. It

serves as a scaffold for proteins in signaling path-

ways involved in sperm maturation, motility, capaci-

tation, hyperactivation and glycolysis. In the

principal piece, the fibrous sheath replaces the

mitochondria sheath. Outer dense fibers 3 and 8

are also substituted by the two longitudinal

columns of the fibrous sheath. AKAP4 recruits PKA

to the fibrous sheath and facilitates local phosphory-

lation to regulate flagellum function. Spermatozoa

from AKAP4= mice lack motility and are infertile

[Miki et al. 2002]. PP1 activity is decreased in the

AKAP4= mice and might indicate the functional

linkage between PP1 and AKAP4 [Huang et al.2005b].

A Hypothesis for the Signaling

Pathways of Mammalian

Spermatozoa Motility Activation

Both the cAMP and Ca2 signal transduction

pathways are involved in activation of motility in

immotile spermatozoa from the cauda epididymis in

rat and mouse [Wade et al. 2003; Schulh et al. 2006].A Ca2 dependent increase in cAMP initiates a signal

transduction cascade for motility activation, which is

independent of protein kinase A-mediated phosphor-

ylation of flagellar proteins in immotile rat spermato-

zoa [Wade et al. 2003]. The concentration of cAMP

increases with activation of motility in spermatozoa

from the cauda epididymis of hamsters [Morton et al.

1974] and rats [Armstrong et al. 1994]. So, cAMP seems

to be the first signal for sperm motility activation.

cAMP-dependent protein kinase (PKA), the major

downstream effector of cAMP signals in sperm, isthen activated and through the AKAPs triggers pro-

tein phosphorylation that might be important for

sperm motility [Si and Okuno 1995; Nolan et al.

2004]. Calmodulin Kinase II (CAMKII) is considered

to be a ubiquitous protein mediating Cai2 signaling

the activation of dynein ATPase in mammalian sperm

[Hsu et al. 2004], and CAMKII-PP1 complex is proven

to act together as a simple device in the Ca2 signal

transduction in the synapses [Bradshaw et al. 2002].

CAMKII can be activated in a persistent manner by

autophospholation at Thr286

. When dephosphory-lated at Thr286 by PP1, CaMKII is deactivated [Nomura

et al. 2004].

Considering activation of the the sperm motility

mechaninism and PP1s=PIPs function during

spermatogenesis we propose the following two-

step signaling pathway of PP1s in controlling sper-

matozoa motility as summarized in Figure 1. In the

Y. Han et al. 174


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first step, motionless spermatozoa in the caput of the

epididymis need to be conditioned through the

AKAP signal transduction pathway which prepares

the microtubules of the spermatozoa to a ready

state for motility activation. In the second step,

motility of the conditioned spermatozoa is then trig-

gered by Ca2 -CAMKII signal transduction pathway

with a functional dynein ATPase. PP1s are involved

in the two signal transduction pathways by interact-

ing with AKAPs and regulating activity of CAMKII,respectively.

ACKNOWLEDGMENT

This work was supported by a direct grant of the

Chinese University of Hong Kong c001-2041219.

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FIGURE 1 cAMP produced by adenylyl cyclase from ATP stimulates the activity of PKA. AKAPs then link the active PKA to both themicrotubules of the spermatozoa flagella and the membrane of the mitochondria. PKA, now in the proximity of the flagella, then stimulates

the microtubule local factors like dynein ATPase by creating target sites of cAMP and/or other unknown functions and makes the

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might stimulate production of more ATP. PP1 is involved in this pathway by interacting with AKAPs. We hypothesize that the binding

of PP1 to the AKAPs might competitively inhibit their binding to PKA. Before binding to AKAPs, the microtubules are not sensitive to

stimulating signals in the absence of key factors. In contrast, the Ca2 CamKII complex could initiate the motility of the conditioned

microtubules by immediately activating dynein ATPase. Motility is maintained by the continuous consumption of ATP. PP1 could

dephosphorylate and inactivate the CAMKII at Thr286. In both pathways, a decrease in the concentration of PP1 would favor the activation

of sperm motility.



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