J Biol Chem, Vol. 275, Issue 13, 9557-9562, March 31, 2000

Interaction of the Somatostatin Receptor Subtype 1 with the Human Homolog of the Shk1 Kinase-binding Protein from Yeast^*

Anja Schwärzler, Hans-Jürgen Kreienkamp, and Dietmar Richter^§

From the Institut für Zellbiochemie und klinische Neurobiologie, Universitätsklinikum Hamburg-Eppendorf, Universität Hamburg, Martinistrasse 52, 20246 Hamburg, Germany

Interaction between the C terminus of a G-protein-coupled receptor and intracellular constituents may represent a crucial step in regulating signal transduction. To identify potential interacting candidates the C terminus of the somatostatin receptor subtype 1 was used as bait in a yeast two hybrid screen of a human brain cDNA library. We identified the human Skb1 sequence (Skb1Hs) as interacting protein, which is homologous to the yeast protein known Skb1 to down-regulate mitosis in Schizosaccharomyces pombe via binding to the Shk1 protein kinase; the latter is a homolog to the mammalian p21^cdc42/Rac-activated protein kinases. Interaction required almost the entire C terminus of the somatostatin receptor subtype 1 including the conserved NPXXY motif of transmembrane region seven; in the case of the Skb1Hs most of the N terminus and an S-adenosylmethionine binding domain were mandatory, whereas the C terminus was not essential. Interaction was verified by coexpression experiments in human embryonic kidney cells. As revealed by immunocytochemical analysis Skb1Hs expressed alone aggregates in large cytosolic clusters. When coexpressed, receptor subtype 1 and Skb1Hs were colocalized at the cell surface; these cells showed a strong increase in somatostatin binding compared with cells expressing the receptor only. This may suggest that Skb1Hs acts like a chaperone by correctly targeting the receptor to the cell surface.