Developmental changes in phosphorylation of MAP-2 and synapsin I in cytosol and taxol polymerised microtubules from...

In cytosol, cyclic AMP stimulated phosphorylation of microtubule associated protein-2 (MAP-2) increased from 2 days to adult in proportion to the increase in the concentration of MAP-2. By contrast, the calmodulin stimulated phosphorylation of MAP-2 decreased in proportion to the decrease in the concentration of calmodulin stimulated protein kinase II (CMK II). Similarly, the cAMP stimulated phosphorylation of the site on synapsin I labeled by the cAMP stimulated protein kinase (PKA) changed little during development whereas the calcium/calmodulin stimulated phosphorylation of the CMK II site decreased dramatically in proportion to the decrease in the concentration of CMK II. The decrease in the concentration of CMK II which occurs in cytosol during synapse maturation was also observed in taxol polymerised microtubules and the effects of the change in the relative concentrations of CMK II and PKA on the phosphorylation of MAP-2 and synapsin I in this fraction were similar to that observed in the cytosol. These results are consistent with the hypothesis that the developmental changes in phosphorylation of endogenous substrates by PKA is controlled largely by changes in the concentration of those substrates, whereas the concentration of CMK II is limiting so that the developmental changes in the phosphorylation of endogenous substrates by CMK II are a function of the concentration of CMK II itself as well as the concentration of endogenous substrates. Some possible functional consequences of this during synapse maturation are discussed.