13.4 g protein-coupled receptors that regulate ion channels by: meredith clement

13.4 G Protein-Coupled 13.4 G Protein-Coupled Receptors That Regulate Ion Receptors That Regulate Ion

ChannelsChannels

By: Meredith ClementBy: Meredith Clement

G Protein ReceptorsG Protein Receptors

Many Neurotransmitters receptors are Many Neurotransmitters receptors are ligand gate ion channels ( Ch. 7) but some ligand gate ion channels ( Ch. 7) but some are G- protein-coupled receptors. The are G- protein-coupled receptors. The effector protein for some of these types of effector protein for some of these types of reactions are Nareactions are Na++ or K or K++ ion channels. The ion channels. The binding of the neurotransmitter to one of binding of the neurotransmitter to one of these receptors causes the channel to these receptors causes the channel to open or close.open or close.

G Protein ReceptorsG Protein Receptors

Other Neurotransmitters and Odorant and Other Neurotransmitters and Odorant and photoreceptors, are G protein receptors photoreceptors, are G protein receptors that indirectly modulate the activity of ion that indirectly modulate the activity of ion channels by the actions of second channels by the actions of second messengers.messengers.

Cardiac Muscarinic Acetylcholine Cardiac Muscarinic Acetylcholine Receptors Activate a G Protein that Receptors Activate a G Protein that

Opens KOpens K++ Channels ChannelsThe Muscarinic Acetylcholine Receptors in The Muscarinic Acetylcholine Receptors in cardiac muscle are inhibitory. The activation of cardiac muscle are inhibitory. The activation of this receptor is coupled to a Gthis receptor is coupled to a G i i protein that leads protein that leads

to the opening of the associated Kto the opening of the associated K++ channels. channels. The influx of the ions causes hyperpolarization The influx of the ions causes hyperpolarization of the plasma membrane. The signal from the of the plasma membrane. The signal from the receptor is transduced to the effector protein by receptor is transduced to the effector protein by the released Gthe released Gβγ βγ subunit rather than the Gsubunit rather than the Gαα·GTP. ·GTP.

The G The Gβγ βγ directly activates the ion channel.directly activates the ion channel.

Figure 13-21 Figure 13-21

GGtt-Coupled Receptors are -Coupled Receptors are

Activated by LightActivated by LightRods and Cones are the primary Rods and Cones are the primary recipients for visual stimulation. recipients for visual stimulation. – Rods are Stimulated by weak light like Rods are Stimulated by weak light like

moonlight.moonlight.– Cones are involved in color vision.Cones are involved in color vision.– All of these signals are interpreted by the All of these signals are interpreted by the

visual cortex in the brain.visual cortex in the brain.


Activated by LightActivated by LightRhodopsin is a G protein that is stimulated by light. It is Rhodopsin is a G protein that is stimulated by light. It is coupled to a trimeric G protein called Transducin (Gt). coupled to a trimeric G protein called Transducin (Gt). Rhodopsin consists of the seven spanning protein opsin Rhodopsin consists of the seven spanning protein opsin to which 11-to which 11-ciscis-retinal( light absorbing protein) is -retinal( light absorbing protein) is bonded. bonded. Upon absorption of light, Rhodopsin is rapidly converted Upon absorption of light, Rhodopsin is rapidly converted to all trans isomers which causes a conformational to all trans isomers which causes a conformational change in the opsin protein that activates it.change in the opsin protein that activates it.This is equivalent the conformation change that occurs This is equivalent the conformation change that occurs upon ligand binding by other G protein –coupled upon ligand binding by other G protein –coupled receptors. receptors. The resulting form of opsin bound to all The resulting form of opsin bound to all transtrans-retinal is -retinal is called Meta-rhodopsin II or Activated opsin.called Meta-rhodopsin II or Activated opsin.

Activated Opsin is unstable and disassociates Activated Opsin is unstable and disassociates back into component parts, releasing opsin and back into component parts, releasing opsin and all all transtrans- retinal. In the dark this is converted - retinal. In the dark this is converted back to 11-back to 11-ciscis –retinal which can rebind with –retinal which can rebind with opsin in order to reform rhodopsin. opsin in order to reform rhodopsin.

Rod cells in the dark are constantly secreting Rod cells in the dark are constantly secreting neurotransmitters. The depolarized state of the neurotransmitters. The depolarized state of the membrane cells is due the presence of open membrane cells is due the presence of open non-selective ion channelsnon-selective ion channels


Activated by LightActivated by Light


Activated by LightActivated by LightAdsorption of light by rhodopsin causes Adsorption of light by rhodopsin causes these channels to close. these channels to close.

The more light absorbed, the more The more light absorbed, the more channels that close. This results in a more channels that close. This results in a more negative environment and fewer negative environment and fewer neurotransmitters being released.neurotransmitters being released.

The human eye is able to see as few as The human eye is able to see as few as five photons.five photons.

Figure 13-23Figure 13-23

Rod Cells Adapt to Varying Levels Rod Cells Adapt to Varying Levels of Ambient Lightof Ambient Light

Visual AdaptationVisual Adaptation– Allows contrasting light to be measure instead of Allows contrasting light to be measure instead of

absolute amounts of light when going from daylight to absolute amounts of light when going from daylight to a dimly lighted room.a dimly lighted room.

– This involves the phosphorylation of activated opsin This involves the phosphorylation of activated opsin by rhodopsin kinase. The more sites that are by rhodopsin kinase. The more sites that are phosphorylated, the less able opsin is to activate Gphosphorylated, the less able opsin is to activate Gt t and thus induce the closing of the cGMP-gated cation and thus induce the closing of the cGMP-gated cation channels.channels.

– When the level of ambient light is reduced, the opsins When the level of ambient light is reduced, the opsins become dephosphorylated and the ability to activate become dephosphorylated and the ability to activate GGtt increases. Resulting in fewer additional photons increases. Resulting in fewer additional photons being necessary to generate a visual signal. being necessary to generate a visual signal.


At high levels of ambient light the level of At high levels of ambient light the level of opsin phosphorylation is such that the opsin phosphorylation is such that the protein β-arrestin binds to the C-terminal protein β-arrestin binds to the C-terminal segment of opsin. This prevents the segment of opsin. This prevents the interaction of Ginteraction of Gtt with activated opsin which with activated opsin which

totally blocks the formation of the active totally blocks the formation of the active GGtαtα·GTP complex. This results in the in the ·GTP complex. This results in the in the

shutdown of all rod cell activity.shutdown of all rod cell activity.


In dark adapted cells the GIn dark adapted cells the Gtαtα and G and Gβγβγ

subunits are in the outer segments but subunits are in the outer segments but exposure for 10 minutes to daytime light exposure for 10 minutes to daytime light causes 80% of the subunits to migrate into causes 80% of the subunits to migrate into other cellular compartments. other cellular compartments. – this results in transducin protein not being able to bind to activated opsin.this results in transducin protein not being able to bind to activated opsin.

13.4 g protein-coupled receptors that regulate ion channels by: meredith clement

Documents

g protein

opsin protein

trimeric g protein

effector protein

spanning protein opsin

gi protein

able opsin

lightactivated opsin