Classification of the Heterotrimeric G Proteins

The important fimctions of signal transmission by G-proteins are realized by the a-subimit. Since different G-proteins interact with very different partners, there are significant differences in the structure of the a-subimits. Due to the common GTPase 

Members of the Gs subfamily are activated by hormone receptors, by odor receptors and by taste receptors. Gg-proteins mediate, e.g., signal transmission by adrenaline receptors of type P and by glucagon receptors. During perception of taste, the taste receptors are activated, which then pass the signal on via the olfactory G-protein Gou. Perception of sweet taste is also mediated via a Gs-protein. Transmission of the 

Oolf nasal epithelium olfactory receptor adenylyl-cyclase [  

The first members of the Gi subfamily to be discovered displayed an inhibitory effect on adenylyl cyclase, thus the name Gi, for inhibitory G-proteins. Further members of the Gi subfamily have phospholipase C as the corresponding effector molecule. Signal transmission via phospholipase C flows into the inositol triphosphate and diacylglyce-rol pathways (see Chapter 6). 

The Gt- and Gg-proteins are also classed as Gi-proteins, based on sequence homologies. The Gt- and Gg-proteins are involved in transmitting sensory signals. Signal transmission in the vision process is mediated via G-proteins known as transducins (Gt). The Gt-proteins are activated by the photoreceptor rhodopsin and are located in the rods and cones of the retina. The sequential effector molecules of the Gt-proteins are cGMP-specific phosphdiesterases (see Fig. 17.9). 

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Table 5.1. Classification of the heterotrimeric G-proteins according to the a-subunits...

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