BETA-2-CHIMAERIN IS A HIGH-AFFINITY RECEPTOR FOR THE PHORBOL ESTER TUMOR PROMOTERS

beta 2-chimaerin, a member of tile GTPase-activating proteins for the small GTP-binding protein p21Rac, possesses a single cysteine-rich dom ain with high homology to those implicated in phorbol ester and diacyl glycerol binding in protein kinase C (PKC) isozymes, We have expressed beta 2-chimaerin in Sf9 insect cells using the baculovirus expression system and determined that, like PKCs, beta 2-chimaerin binds phorbol esters with high affinity in the presence of phosphatidylserine as a cofactor, Scatchard plot analysis using the radioligand [H-3]phopbol 1 2,13-dibutyrate revealed a dissociation constant of 1.9 +/- 0.2 nM for beta 2-chimaerin. Likewise, beta 2-chimaerin is a high affinity recep tor for the bryostatins, a class of atypical PHC activators. A detaile d comparison of structure activity relations using several phorbol est er analogs revealed striking differences in binding recognition betwee n beta 2-chimaerin and PKC alpha, Although the diacylglycerol 1-oleoyl -2-acetylglycerol binds with similar potency to both beta 2-chimaerin and PKC alpha; the mezerein analog thymeleatoxin has 56-foId less affi nity for binding to beta 2-chimaerin, To establish whether beta 2-chim aerin responds to phorbol esters in cellular sys-- tems, we overexpres sed beta 2-chimaerin in COS-7 cells and monitored its subcellular dist ribution after phorbol ester treatment, Interestingly, as described pr eviously for PI(C isozymes, beta 2-chimaerin translocates from cytosol ic to particulate fractions as a consequence of phorbol ester treatmen t. Our results demonstrate that beta 2-chimaerin is a novel target for the phorbol aster tumor promoters. The expansion of the family of pho rbol ester receptors strongly suggests a potential for the ''nonkinase '' receptors as cellular mediators of the phorbol ester responses.