SMAP, an Smg GDS-associating protein having arm repeats and phosphorylated by Src tyrosine kinase

Smg GDS is a regulator having two activities on a group of small G proteins including the Rho and Rap1 family members and Ki-Ras; one is to stimulate their GDP/GTP exchange reactions, and the other is to inhibit their interactions with membranes. Structurally, it has 11 Arm repeats, a protein interaction motif, found in the Drosophila Armadillo protein, a homolog of mammalian beta-catenin. We have isolated here an Smg GDS-interacting protein from a human brain cDNA library by use of the yeast two-hybrid method and named it SMAP (Smg GDS-associated protein). SMAP was a protein with a Mr of 91,189 and 792 amino acids. SMAP had 9 Arm repeats. Recombinant SMAP interacted with recombinant Smg GDS but did not affect the two activities of Smg GDS on RhoA. SMAP was tyrosine phosphorylated by v-Src, and this phosphorylation reduced the affinity of SMAP for Smg GDS. Tissue and subcellular distribution analyses indicated that SMAP was ubiquitously expressed and highly concentrated at the endoplasmic reticulum area. Searches for sequence homology to SMAP revealed that SMAP was significantly homologous to sea urchin SpKAP115, suggesting that SMAP is a mammalian counterpart of SpKAP115 or its related protein. SpKAP115 is an accessory subunit of sea urchin kinesin II, an ATPase motor that transports vesicles along microtubules. These results suggest that SMAP serves as an adaptor for both Smg GDS and kinesin II or its related protein and links them with both the Smg GDS-regulated small G protein and Src tyrosine kinase signalings.