Calcium/calmodulin-dependent protein kinase II phosphorylates tau at Ser-262 but only partially inhibits its binding to microtubules

PHF-tau, which is phosphorylated at 10 Ser/Thr-Pro and 11 non-Ser/Thr-Pro sites, is unable to promote microtubule assembly, Phosphorylation of the non-Ser/Thr-Pro site, Ser-262, is reported to be primarily responsible for this, The identities of kinase(s) responsible for Ser-262 phosphorylation are still to be clarified, In this study we have used the monoclonal antibody 12E8, which recognizes P-Ser-262 and P-Ser-356 on tau, to survey different kinases for their abilities to phosphorylate Ser-262 on human tan 3L (tau(410)). In decreasing order of effectiveness we found that Ser-262 and Ser-356 phosphorylation A can be catalyzed by CaM kinase II much greater than C-kinase much greater than GSK-3 congruent to A-kinase much greater than CK-1. CaM kinase II and C-kinase were shown to phosphorylate both Ser-262 and Ser-356. The binding of tau to taxol-stabilized microtubules was decreased by 35 and 42% after phosphorylation by CaM kinase II and C-kinase, respectively, Of the fraction of tau that hound to microtubules, about 50% was phosphorylated at Ser-262 and Ser-356. These results suggest that Ser-262 and Ser-356 are very good substrates for CaM kinase II but their phosphorylations are not sufficient to achieve maximal inhibition of tau binding to microtubules.