THE 86-KDA SUBUNIT OF AUTOANTIGEN KU IS A SOMATOSTATIN RECEPTOR REGULATING PROTEIN PHOSPHATASE-2A ACTIVITY

We previously reported the immunopurification of a somatostatin recept or from the human tumoral gastric cell HGT1 using the monoclonal antib ody 30F3 (Reyl-Desmars, F., Le Roux, S., Linard, C., Benkouka, F., and Lewin, M. J. M. (1989) J. Biol. Chem. 264, 18789-18795). Screening of a lambda gt11 HGT1-cDNA library with 30F3 led us to isolate a cDNA en coding an 86-kDa polypeptide displaying 100% structural identity with the 86-kDa subunit (p86-Ku) of the Ku autoantigen. Recombinant p86 exp ressed in Escherichia coli cross-reacted with 30F3 and specifically bo und [I-125-Tyr(11)]somatostatin-14. Binding was totally displaced by s omatostatin-14, somatostatin-28, and SMS 201-995, with IC50 values of 0.7, 1.0, and 1.2 nM, respectively. In a search for a biological effec t associated with binding, we purified a 36-kDa, okadaic acid-sensitiv e phosphatase (protein phosphatase-2A (PP2A)) from rat gastric cytosol . PP2A catalyzed P-32 release from p34(cdc2)-phosphorylated histone H1 . However, PP2A-induced P-32 release was concentration dependently inh ibited by recombinant p86-Ku, with a decrease in maximal velocity with out a change in K-m. Steric exclusion high pressure chromatography ind icated that the inhibition resulted from direct interaction of the enz yme with p86-Ku. Furthermore, it was antagonized by increased concentr ations of somatostatin-14 and prevented by preincubating p86-Ku with 3 0F3. Given the key role played by PP2A in cell cycle regulation, the c urrent findings suggest that p86-Ku could be a physiological target of somatostatin antiproliferative action.