GAIP, A PROTEIN THAT SPECIFICALLY INTERACTS WITH THE TRIMERIC G-PROTEIN G-ALPHA(I3), IS A MEMBER OF A PROTEIN FAMILY WITH A HIGHLY CONSERVED CORE DOMAIN

Using the yeast two-hybrid system we have identified a human protein, GAIP (G Alpha Interacting Protein), that specifically interacts with the heterotrimeric GTP-binding protein G alpha(i3). Interaction was verified by specific binding of in vitro-translated G alpha(i3) with a GAIP-glutathione S-transferase fusion protein. GAIP is a small protein (217 amino acids, 24 kDa) that contains two potential phosphorylation sites for protein kinase C and seven for casein kinase 2. GAIP shows high homology to two previously identified human proteins, GOS8 and 1R20, two Caenorhabditis elegans proteins, C05B5.7 and C29H12.3, and the FLBA gene product in Aspergillus nidulans-all of unknown function. Significant homology was also found to the SST2 gene product in Saccharomyces cerevisiae that is known to interact with a yeast G alpha subunit (Gpa1). A highly conserved core domain of 125 amino acids characterizes this family of proteins. Analysis of deletion mutants demonstrated that the core domain is the site of GAIP's interaction with G alpha(i3). GAIP is likely to be an early inducible phosphoprotein, as its cDNA contains the TTTTGT sequence characteristic of early response genes in its 3'-untranslated region. By Northern analysis GAIP's 1.6-kb mRNA is most abundant in lung, heart, placenta, and liver and is very low in brain, skeletal muscle, pancreas, and kidney. GAIP appears to interact exclusively with G alpha(i3), as it did not interact with G alpha(i2) and G alpha(q). The fact that GAIP and Sst2 interact with G alpha subunits and share a common domain suggests that other members of the GAIP family also interact with G alpha subunits through the 125-amino-acid core domain.