A neuroblastoma protein related to the brain microtubule-associated protein, MAP-1B, as determined by immunoprecipitation and coassembly with brain microtubules, becomes phosphorylated when N2A mouse neuroblastoma cells are induced to generate microtubule-containing neurites. To characterize the protein kinases that may be involved in this in vivo phosphorylation of MAP-1B, we have studied its in vitro phosphorylation. In brain microtubule protein, MAP-1B appears to be phosphorylated in vitro by an endogenous casein kinase II-like activity which also phosphorylates the related protein MAP-1A but scarcely phosphorylates MAP-2. A similar kinase activity has been detected in cell-free extracts of differentiating N2A cells. Using brain MAP preparations devoid of endogenous kinase activities and different purified protein kinases, we have found that MAP-1B is barely phosphorylated by cAMP-dependent protein kinase, Ca/calmodulin-dependent protein kinase, or Ca/phospholipid-dependent protein kinase whereas MAP-1B is one of the preferred substrates, together with MAP-1A, for casein kinase II. Brain MAP-1B phosphorylated in vitro by casein kinase II efficiently coassembles with microtubule proteins in the same way as in vivo phosphorylated MAP-1B from neuroblastoma cells. Furthermore, the phosphopeptide patterns of brain MAP-1B phosphorylated in vitro by either purified casein kinase II or an extract obtained from differentiating neuroblastoma cells are identical to each other and similar to that of in vivo phosphorylated neuroblastoma MAP-1B. Thus, we suggest that the observed phosphorylation of a protein identified as MAP-1B during neurite outgrowth is mainly due to the activation of a casein kinase II-related activity in differentiating neuroblastoma cells. This kinase activity, previously implicated in beta-tubulin phosphorylation (Serrano, L., J. Díaz-Nido, F. Wandosell, and J. Avila, 1987. J. Cell Biol. 105: 1731-1739), may consequently have an important role in posttranslational modifications of microtubule proteins required for neuronal differentiation.

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