The occurrence of microtubule-associated proteins 1 and 2 in a synaptic junction preparation from rat cerebrum.

SJ proteins termed P1, P2, P3 and P4 were found to have molecular weights very close to MAP1 or MAP2. P1 and P2 reacted with an antibody to the combined antigens, MAP1a, MAP1b and MAP1c, but neither one of the peptide maps of P1, P2, nor P3 resembled either one of the maps of MAP1a plus MAP1b, MAP1c, MAP2a or MAP2b. Thus, proteins 1 and 2 were immunologically related to, but structurally different from, MAP1 subspecies. The protein P4 reacted with an antibody to combined MAP2a and MAP2b, and a peptide map of P4 resembled those of MAP2a and MAP2b. Thus, P4 is probably identical to MAP2. P1, P2, P3 and P4 appeared to be concentrated in SJ as compared to synaptosomes or a synaptic plasma membrane fraction, although the contents of these proteins, especially P1 and P4, were small. P2 (MAP1-related protein) was phosphorylated endogenously both in the presence and in the absence of CaC1(2) plus calmodulin, while the MAP2-related protein P4 in SJ was hardly phosphorylated by the endogenous kinases.