COPURIFICATION OF A PROTEIN-TYROSINE-PHOSPHATASE WITH ACTIVATED SOMATOSTATIN RECEPTORS FROM RAT PANCREATIC ACINAR MEMBRANES

We have previously shown that somatostatin promotes the stimulation of a membrane tyrosine phosphatase activity in pancreatic cells. To gain insight into the mechanism of somatostatin action, we purified somato statin-receptor complexes from somatostatin 28-prelabelled rat pancrea tic plasma membranes by immunoaffinity chromatography using immobilize d antibodies raised against the N-terminal part of somatostatin 28, so matostatin 28 (1-14), which is not involved in receptor-binding-site r ecognition. After SDS gel electrophoresis a band with a molecular mass of 87 kDa was identified in the affinity-purified material as the som atostatin receptor. The 87 kDa protein was not observed when the membr ane receptors were solubilized in a free unoccupied or somatostatin 14 -occupied form, or when nonimmune serum replaced the anti-[somatostati n 28 (1-14)] antiserum. Somatostatin 14 inhibited the appearance of th e 87 kDa protein in the same range of concentrations that inhibit radi oligand binding on pancreatic membranes. After somatostatin 28 treatme nt of membranes, purified somatostatin receptor preparations exhibited an elevated tyrosine phosphatase activity that dephosphorylated phosp horylated epidermal growth factor receptor and poly(Glu,Tyr). This act ivity was related to the presence of somatostatin receptors in purifie d material. It was increased by dithiothreitol and inhibited by orthov anadate. In purified material containing somatostatin receptors, anti- [Src homology 2 domains (SH2)]-containing tyrosine phosphatase SHPTP1 polyclonal antibodies identified a protein of 66 kDa which was not det ected in the absence of somatostatin receptor. Furthermore, the anti-S HPTP1 antibodies immunoprecipitated specific somatostatin receptors fr om somatostatin-prelabelIed pancreatic membranes and from untreated me mbranes. These results indicate that a 66 kDa tyrosine phosphatase rel ated to SHPTP1 co-purifies with the pancreatic somatostatin receptors, and suggest that this protein is associated with somatostatin recepto rs at the membrane level.