Interactions between G proteins and downstream effectors

Most of the information about the effector binding domains of Ga subunits is obtained from the construction of chimeric proteins and subsequent site-directed mutagenesis targeted to identify specific effector interacting residues. The C-terminal portion of Gas (residues 235-356) contained the AC-activating domain (Osawa et al. 1990). Mutations within this region ofGas decreased its ability to stimulate AC (ItohandGilman 1991). The AC activating domain is comprised of three discrete stretches of residues of Gas (Berlot and Bourne 1992), corresponding to three of the loops (a2/|4, a3/|5, and a4/|6 loops) observed on Ga crystals. The Switch IV region (aB/aC junction) may also be a crucial but indirect element for AC stimulation (Echeverría etal. 2000).

Similarly, other Ga-effector interaction surfaces have also mapped to comparable regions. The AC inhibiting residues of Ga¡2 are located at the Switch II region and the a4/|6 loop. Similar regions are also identified as the AC-interacting domains of Gaz, suggesting that both Ga¡2 and Gaz may act on the same contact sites of AC (Ho and Wong 2000). For Gaq, discrete regions between a3 helix and |6 strand and the N-terminal cysteine residues are essential for PLC activation. For Gat1, the a2, a3 and a4 helices and the loops followed form the interacting surfaces for the y subunit of cGMP phosphodiesterase (PDEy) (for details, see Hamm 1998).

It has been shown that tubulin associated with the synaptic membrane can interact with Gs and G¡2 (Wang et al. 1990). For Gas, residues 54-212 are important for the GTP transfer from tubulin, while the first 54 amino acids are required for the ability of tubulin to stimulate AC(Popova etal. 1994). It has been postulated that the active Gas conformation provoked by nucleotide transfer from tubulin is stabilized by Gas-tubulin interaction leading to extended stimulation of AC. Gas suppresses the commitment of 3T3-L1 cells to adipocytes in response to dexamethasone, but Gai2 enhances the adipogenesis. The process is independent to AC regulation (Wang and Malbon 1996) and the amino acids 146-220 of Gas covering Switches I and II are likely the suppressor domain for adipogenesis (Liu etal. 1998).

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