PROTEIN-KINASE-C AND CALCIUM CALMODULIN-DEPENDENT PROTEIN-KINASE-II PHOSPHORYLATE 3-REPEAT AND 4-REPEAT TAU-ISOFORMS AT DIFFERENT RATES/

All six isoforms of the microtubule-associated protein tau are present in hyperphosphorylated states in the brains of patients with Alzheime r's disease (AD). It is presently unclear how such hyperphosphorylatio n of tau is controlled. In a previous study (Singh et al. Arch Biochem Biophys 328: 43-50, 1996) we have shown that three-repeat taus contai ning two N-terminal inserts were phosphorylated to higher levels and a t different sites compared to those either lacking or containing only one such insert. We have extended these observations in this study by comparing the phosphorylation of tau isoforms containing three-repeats (tau 3,tau 3L) and four-repeats (tau 4,tau 4L). In the absence of N-t erminal inserts in tau structure (tau 3,tau 4) both CaM kinase II and C-kinase phosphorylated four-repeat tau (tau 4) to a higher extent tha n three-repeat tau (tau 3). When two N-terminal inserts are present in tau structure (tau 3L,tau 4L), then three-repeat tau (tau 3L) is phos phorylated to a higher extent than four-repeat tau (tau 4L) by these k inases, CK-I and GSK-3 phosphorylated each of the above pairs of three -repeat and four-repeat taus to the same extents. However, after an in itial prephosphorylation of the taus by CaM kinase II, GSK-3 different ially phosphorylated three-repeat and four-repeat taus. Under these co nditions thr 231, ser 235, ser 396, and ser 404 were phosphorylated to greater extents in four-repeat tau (tau 4) compared to three-repeat t au (tau 3) in the absence of N-terminal inserts. In the presence of su ch inserts these sites were phosphorylated to greater extents in three -repeat (tau 3L) compared to four-repeat (tau 4L) tau. Our results ind icate that the extents to which tau isoforms are phosphorylated in nor mal and AD brain depends on (a) the number of repeats(3 or 4), (b) the number of N-terminal inserts (0, 1,or 2), and (c) the initial phospho rylation state of tau.