| To define the role of regulators of G protein signaling ( RGS ) in chemoattractant mediated responses , RGS 4 and the receptors for platelet activating factor ( PAFR ) , formylated peptides ( FR ) , or interleukin 8 ( CXCR 1 ) were stably coexpressed in a rat basophilic leukemia ( RBL 2H3 ) cell line . ^^^ The data demonstrate that RGS 4 inhibited responses by PAFR ( i . e . , phosphoinositide ( PI ) hydrolysis , Ca2+ mobilization ) but not by FR or CXCR 1 . ^^^ An N terminal 33 amino acid deletion mutant of RGS 4 ( DeltaRGS 4 ) , deficient in GAP ( GTPase activating protein ) activity and plasma membrane localization , had no effect on either PAFR , FR , or CXCR 1 . ^^^ RGS 4 , but not DeltaRGS 4 , also blocked phosphorylation of PAFR by platelet activating factor ( PAF ) and , unexpectedly , by phorbol 12 myristate 13 acetate ( PMA ) ; it also blocked cross phosphorylation by formylmethionylleucylphenylalanine ( fMLP ) . ^^^ A point mutant of RGS 4 ( N88S ) , deficient in GAP activity but not membrane localization , partially blocked PAFR phosphorylation but had no effect on PAFR mediated PI hydrolysis and Ca2+ mobilization . ^^^ |