A NOVEL FUNCTIONAL TARGET FOR TUMOR-PROMOTING PHORBOL ESTERS AND LYSOPHOSPHATIDIC ACID - THE P21RAC-GTPASE ACTIVATING PROTEIN N-CHIMAERIN

Phorbol esters are potent tumor promoters widely used for investigatin g mechanisms of cell transformation with protein kinase C (PKC) genera lly considered as being their only protein target. Lysophosphatidic ac id (LPA) can act as a mitogen, affecting cell shape and the actin cyto skeleton. There is no identified functional target for LPA. We have is olated a cDNA encoding a protein n-chimaerin that is a high affinity p horbol ester receptor and a p21rac-GTPase activating protein (rac-GAP) . p21rac is a member of the ras superfamily of small molecular weight GTP-binding proteins, which stimulates actin microfilament formation i n Swiss 3T3 cells and superoxide production by the neutrophil oxidase. We now show that the rac-GAP activity of n-chimaerin is stimulated by phosphatidylserine (PS) and phosphatidic acid (PA) and that phorbol e sters can synergize with PS and PA. LPA, in contrast, was found to inh ibit n-chimaerin. The phospholipid/phorbol ester modulation of the rac -GAP activity requires the PKC-like cysteine-rich domain of n-chimaeri n. Thus, n-chimaerin is a novel functional target (distinct from PKC) for both phorbol esters and LPA. These data suggest that the physiolog ical role of n-chimaerin is to link events initiating at the cell surf ace/membrane with p21rac effector pathways.