1. G Proteins - Part 2 Biochemistry 4000 Dr. Ute Kothe

2. Ras-activated MAP kinase cascade Voet, Biochemistry

3. Heterotrimeric G proteins Voet, Biochemistry 90º three subunits: , associate only in GDP form  subunit = GTPase  &  subunits are linked to membrane by covalently attached lipids become activated by membrane receptor (GEF) become inactivated by “special GAPs” called Regulators of G protein signaling (RGS)

4. G  proteins Voet, Biochemistry at least 27 different G  proteins in humans three Switch regions extra helical domain inserted into canonical G domain G t  (transducin) – involved in vision

5. G protein coupled Receptors Voet, Biochemistry G protein coupled receptors (GPCR): 7 transmembrane receptors GEF for heterotrimeric G proteins activation of G  induces dissociation of G  large class of receptors, e.g. for epinephrine (adrenergic receptors), odorant molecules, light (rhodopsin)

6. Activation & Inhibition of Adenyl Cyclase (AC) by G  proteins Voet, Biochemistry

7. G i  1 -RGS4 complex Tesmer, Cell 1997 G i  1 -Mg 2+ -GDP-AlF 4 - -RGS4 2.8 Å, molecular replacement using G i  1 -GDP-AlF 4 - structure R cryst = 29%, R free = 21% RGS4 contacts all switch regions of G i  1 with the bottom of its four-helix bundle

8. Interface between RGS4 & G i  1 Tesmer, Cell 1997 RGS4 loop 3-4, loop 5-6 and loop 7-8 interact with all three switch regions of Gi  1 RGS4 reduces the flexibility of all three switch regions (comparison of B factors of this structure with G i  1 alone)

9. Active Site of G i  1 Tesmer, Cell 1997 no catalytic residue added by RGS4 GTPase activation by stabilization of the switch regions, i.e. stabilization of the transition state RGS4-Asn128 might facilitate GTP hydrolysis either by stabilizing Gln 204 or by orienting the attacking H 2 O catalytic Arginine 178 provided in cis

10. Visual preception Lodish, 5th edition Rhodopsin is activated by light Acts as GEF for transducin (G  only found in rod cells)

11. Phototransduction cascade Lodish, 5th edition  Rhodopsin  Transducin (Gta)  cGMP phosphodiesterase (PDE)  cGMP GMP  cGMP dissociates from nucleotide-gated channel  Channel closes  Hyperpolarization of membrane

12. Light Detection by Retinal Lodish, 5th edition Retinal is covalently bound to Rhodopsin Isomerization of Retinal results in conformational change of rhodopsin (activation)

13. Organization of the olfactory system Olfactory cortex Main olfactory bulb Sensory neurons in Olfactory ephithelium Nasal cavity

14. Olfactory signal transduction Odorant Receptors: discovered in 1991  Nobel prize for Linda Buck & Richard Axel 2004 GPCRs largest multigene family ~ 350 in human ~ 1000 in mice (1-3 % of the genes)