MECHANISM OF CAMP-INDUCED CA2- ROLE OF PHOSPHOLIPASE A(2)( INFLUX IN DICTYOSTELIUM )

cAMP-induced Ca2+ influx in Dictyostelium follows two pathways: a G-pr otein-dependent pathway where influx is reduced by 50-70% in G alpha 2 and G beta-negative strains and a heterotrimeric G-protein-independen t pathway. Using a pharmacological approach, we found that phospholipa se A(2) (PLA(2)) is the target of both pathways. The products of PLA(2 ) activity, arachidonic acid (AA) and palmitic acid, induced Ca2+ infl ux to a similar extent as cAMP. Half-maximal activation occurred at 3 mu M AA and saturation at 10 mu M AA. The response to AA was quantitat ively similar throughout early differentiation and thus independent of cAMP-receptor concentration. Synergy experiments revealed that cAMP a nd AA acted through identical pathways. The PLA(2)-activating peptide, a peptide with sequence similarity to the G-protein beta-subunit, act ivated Ca2+ flux. The G-protein-independent pathway was sensitive to g enistein but not to blockers of protein kinase C and other kinases, su ggesting that tyrosine kinase may directly or indirectly activate PLA( 2) in this case.