AHNAK, a protein that binds and activates phospholipase C-gamma 1 in the presence of arachidonic acid

We have recently shown that phospholipase C-gamma (PLC-gamma) is activated by tau, a neuronal cell-specific microtubule-associated protein, in the pre sence of arachidonic acid, We now report that non-neuronal tissues also con tain a protein that can activate PLC-gamma in the presence of arachidonic a cid. Purification of this activator from bovine lung cytosol yielded severa l proteins with apparent molecular sizes of 70-130 kDa. They were identifie d as fragments derived from an unusually large protein (similar to 700 kDa) named AHNAK, which comprises about 30 repeated motifs each 128 amino acids in length. Two AHNAK fragments containing one and four of the repeated mot ifs, respectively, were expressed as glutathione S-transferase fusion prote ins. Both recombinant proteins activated PLC-gamma 1 at nanomolar concentra tions in the presence of arachidonic acid, suggesting that an intact AHNAK molecule contains multiple sites for PLC-gamma activation. The role of arac hidonic acid was to promote a physical interaction between AHNAK and PLC-ga mma 1, and the activation by AHNAK and arachidonic acid was mainly attribut able to reduction in the enzyme's apparent K-m toward the substrate phospha tidylinositol 4,5-bisphosphate. Our results suggest that arachidonic acid l iberated by phospholipase A(2) can act as an additional trigger for PLC-gam ma activation, constituting an alternative mechanism that is independent of tyrosine phosphorylation.