LOCALIZATION OF DIFFERENTIALLY PHOSPHORYLATED ISOFORMS OF MICROTUBULE-ASSOCIATED PROTEIN 1B IN CULTURED RAT HIPPOCAMPAL-NEURONS

The development and plasticity of axons and dendrites in mammalian neu rons may depend on the presence and phosphorylation state of cytoskele tal proteins, including certain microtubule-associated proteins. One o f these proteins, microtubule-associated protein 1B, is modified by di fferent protein kinases, which give rise to two major types of phospho rylated isoforms. The distribution of these isoforms in cultured hippo campal neurons has been studied using antibodies to specific phosphory lation-sensitive epitopes. Mode I-phosphorylated MAP1B is largely rest ricted to developing axonal processes, particularly at their distal re gions including their growth cones where no mode I-dephosphorylated MA P1B is present. Axonal maturation is accompanied by dephosphorylation of MAP1B at mode I sites. Thus, mode I-phosphorylated MAP1B may be a m arker for axonal growth. In contrast, mode II-phosphorylated MAP1B is abundant in the axonal and somatodendritic compartments, and no increa sed dephosphorylation occurs during maturation. These results are comp atible with a role for the mode I phosphorylation of MAP1B (which migh t be catalysed by proline-directed protein kinases) in supporting a ra pid axonal-specific growth mechanism and a more general role for the m ode II phosphorylation of MAP1B (which seems to be catalysed by casein kinase II) in controlling axonal and dendritic growth and remodeling.