Cell Signaling (BIO-203) Lecture 4

How the signaling terminates The G α -GTP state is short-lived because the bound GTP is hydrolyzed to GDP in minutes. The GPCR signal-transduction system contains a built-in feedback mechanism that ensures the effector protein becomes activated only for a few seconds or minutes following receptor activation. G α the switches back to G α -GDP state blocking any further activation of effector proteins. The resulting G α -GDP reassociates with Gβγ and the complex becomes ready to interact with an activated receptor to stop the process.

Light activates G α t - Coupled rhodopsins Human retina contains 2 types of photoreceptor cells: Rods stimulated by moonlight over a range of wavelengths. Cones involved in color vision. They are the primary recipients of visual stimulation. Rhodopsin consists of the protein opsin which has a usual GPCR structure covalently bonded to light-absorbing pigment 11-cis- retinal.

The trimeric G Protein couple to rhodopsin is called transducin (Gt). It contains G α t subunit. Rhodopsin and G α t subunit are found only in rod cells.

Rhodopsin is sensitive enough to respond to a single photon of light, this response takes place in the form of isomeriztion, Therefore, when a photon of light enters the eye, it is absorbed by the retinal and causes a change in its configuration from 11-cis retinal to all-trans retinal. This isomeriztion induces conformational changes in Rhodopsin that activates the G-protein.

Isomerization is the process by which one molecule is transformed into another molecule which has exactly the same atoms, but the atoms are rearranged e.g. A-B-C → B-A- C.

Light activated rhodopsin pathway In dark adopted rod cells: Light absorption generated activated opsin Opsin binds inactive GDP-bound G α t protein and mediates replacement of GDP with GTP The free G α t-GTP activates cGMP phosphodiesterase (PDE) by binding to its inhibitory γ subunits and dissociating them from the catalytic α and β subunits. The free α and β subunits convert cGMP to GMP. The resulting decrease in cGMP leads to dissociation of cGMP from the nucleotide-gated channels in the plasma membrane and closing of channels. The membrane then becomes hyperpolarized.

Light activated rhodopsin pathway

GPCRs that activate adenylyl clase Following ligand binding to the receptor, the Gs protein relays the hormone signal to the effector protein,, adenylyl cyclase. Gs cycles between an inactive form with bound GDP and an active form with bound GTP. Dissociation of the active form yields the Gs α · GTP complex, which directly activates adenylyl cyclase. Activation is short-lived because GTP is rapidly hydrolyzed (step 5 ). This terminates the hormone signal and leads to reassembly of the inactive Gs · GDP form, returning the system to the resting state. Binding of another hormone molecule causes repetition of the cycle. Both the G γ and Gs α subunits are linked to the membrane by covalent attachment to lipids. Binding of the activated receptor to Gs α promotes dissociation of GDP and its replacement with GTP.

GPCRs that activate and inhibit adenylyl cyclase In the liver, glucagon and epinephrine bind to different GPCRs, but binding of both hormones activates adenylyl cyclase and thus triggers the same metabolic responses. Both types of receptors interact with and activate Gs, converting the inactive Gs · GDP to the active Gs α · GTP form. Activation of adenylyl cyclase, and thus the cAMP level, is proportional to the total concentration of Gs α · GTP resulting from binding of both hormones to their respective receptors. In some cells, the cAMP level can be both up-regulated and down-regulated by the action of different hormones. Prostaglandin PGE1 and adenosine inhibit the enzyme. The receptors for PGE1 and adenosine interact with inhibitory Gi, which contains the same β and γ subunits as stimulatory Gs but a different α subunit (Gi α ). In response to binding of an inhibitory ligand to its receptor, the associated Gi protein releases its bound GDP and binds GTP; the active Gi α · GTP complex then dissociates from G βγ and inhibits (rather than stimulates) adenylyl cyclase.

GPCRs that activate and inhibit adenylyl cyclase