Cell Signaling in Space and Time: Where Proteins Come Together and When Theyre Apart

Cell Signaling in Space and Time:Where Proteins Come Together andWhen Theyre Apart

Cells typically receive chemical signals via signaling molecules and integrate the information they receive. receptorswhich are transmembrane proteins specifically bind the signaling molecule and initiate a signal transmission through a sequence of molecular switches to the cell interior. for all of these signaling pathways, information is conveyed either through proteinprotein interactions or it is transmitted by diffusible elements usually referred to as second messengers

Requirements for Processing of information Cells have developed four regulatory mechanisms to ensure all the elements needed for processing the information come together at the right time and right place. Signal dependent formation of protein complexesProcessing of signals through preassembled protein complexesEnzyme regulation by subcellular localizationTemporal control of signaling pathway

Protein complex formation requires Post translational modifications such as phosphorylation, phosphoinositides, ubiquitination, and acetylation, which allows communication between cells.

SH2 domainSRC Homology 2 (SH2) domain :sequence-specific phosphotyrosine-binding moduleDiscovered in P130Gag-Fps oncoproteinContains 100 amino acids which bind to pYAdapter proteins contain several domains such as the SH2 and SH3 domains within their structure, thus allowing interactions with other specific proteins. SH2 domains bind to receptor tyrosine kinase and SH3 domain recognizeproline-rich sequences

SH2 domain interactions with SH3 domain can lead to Kinase inactivation. SH2-SH3 unit block the activity of Abl, Src, Lyn, Fyn. They act by stabilizing the enzyme inactive conformation and suppressing its activity. SH2 domain have the ability to discriminate between adjacent residues and recognize different phosphorylated sites.

NcK adapter proteinNcK adapter protein consists of one SH2 domain and three SH3 domains. It links receptor and nonreceptor tyrosine kinases to actin cytoskeleton reorganizing proteins, they are involved in cell movements and axon guidance. Stabilizing interactions between the SH2 and the kinase domains have also been observed in FES SH2 functional unit

Grb2 adaptor proteinGrb2 adaptor protein and effector proteins such as phosphatidylinositol 3-kinase (PI3K) or phospholipaseCg (PLC-g) are recruited when receptor tyrosine kinase becomes auto-phoshphorylated which causes progression of cancer cells.

Histone Ubiquitinationpost-transcriptional modification which plays an essential role in regulation of all DNA related processesPoly ubiquitination: 76 amino acid polypeptide attached to histone lysines and signal proteins for degredation via 26S proteosomeUbiquitin-binding domains (UBDs) are a collection of modular protein domains that non-covalently bind to ubiquitin. Because these bindings are readily reversible UBD motifs interpret and transmit information and control various cellular events such as endosomal sorting, vesicular trafficking,

AcetylationAcetylation of lysine residue is highly dynamic and is controlled by two families of enzymes:1) HATs: Histone acetyl transferase- utilize acetyl- CoA as a cofactor. Catalyze transfer of an acetyl group to the e-amino group of lysine side chains. Ubiquitination of the same side chain can be prohibited by lysine acetylation. For example when the cell DNA is damaged, tumor suppressor protein P53 is stabilized by acetylation results in protection of P53 from degradation from ubiquitin- proteosome. 2) HDACs: Histone deacetylases- have relatively low substrate specificity. A single enzyme can de-acetylate multiple sites within histones.

Protein Complexes Signal Integration

How is this achieved?Think of the overall machinery as an automated car wash.Car represents substrateThe car needs to follow a series of steps in a predetermined sequence in order to be washed properly.

Signal processing through protein complexesLinear flow of signaling informationEasier to manage protein-substrate interactionsImproves fidelity of cell signalOverall efficiency of a signal pathway increases

Efficiency & Fidelity By clustering all the instruments required for substrate processing the process becomes more efficient and faithful.Members of the protein complex will only process the previous substrate.Ex: on a 3 member protein complex 2 will only process the product of 1, and three the product of 2.

Some examples:MAPKs (mitogen-activated protein kinases)JNKs (Jun N-terminal Kinase)

MAP-Kinase (mitogen-activated protein) Signaling Module

Linear flow of signalsGrowth Factor Stimulates RTKs (receptor tyrosine kinases) Starts signalRas (GTP-binding protein) forms complex with SOS and Grb2 protein to form the active form of RasThis allows MAP KKK to phosphorylate MEK1/2 (MAP KK) and then MEK1/2 phosphorylates ERK1/2 (MAP K) ERK1/2 is then free to phosphorylate different combinations of proteins and begin transcriptional regulationhttp://dna.brc.riken.jp/en/GENESETBANK/0200104Erk_cascade_2.html

Temporal Control of Signaling PathwaysChanges in the composition or the amount of enzyme complexes over time modulate the cellular event Ex: phosphorylation , degradation by ubiquitation or the translocation of signaling components.

Nuclear factor kB is transcription factor that regulates expression of genes involved in inflammation, apoptosis and tumorigenesis IkB kinases complex phosphorylation ubiquitination degradation translocation to the nucleus

Hypoxia-inducible factor 1 (HIF-1 )Is induced in cardiomyocytes and tumor in response to reduced intracellular oxygen

In the normoxic condition, the concerntration is low beacause of its ubiquitin-mediated proteosomal degradation. In the hypoxia, enzymatic activity of PHD halt the destruction of the HIF-1 , and E3 ligase ubiquitinates and degrades the HIF-1 . The degraded HIF-1 is translocated to the nucleus and combine with the HIF-1 to form a heterodimetric complex to initiate the transcription of proangiogenic, metabolic, and antiapoptotic genes that promote cell survivial