Sequential phosphorylation of chartin microtubule-associated proteins is regulated by the presence of microtubules.

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Abstract
Chartins are a unique class of three families of microtubule-associated proteins, each consisting of several isoforms possessing varying degrees of phosphorylation. The most highly phosphorylated chartin isoforms are highly enriched in neuronal cell fractions containing microtubules and there is evidence that their phosphorylation may play a role in promoting neurite outgrowth. The present work describes the relationship between the phosphorylation state of chartins and the presence of intact microtubules in long-term cultures of NGF-treated, neurite-bearing PC12 cells. Cultures were depleted of microtubules by exposure to high concentrations of depolymerizing agents for 2-24 h. Radiolabeling of cellular proteins with [32P]orthophosphate or [35S]methionine revealed that both the ongoing and steady-state phosphorylation of chartins is markedly altered under these conditions. Two-dimensional isoelectric focusing by SDS-PAGE of whole cell extracts demonstrated that the more acidic, highly phosphorylated isoforms are diminished with a concomitant increase in the more basic, less phosphorylated isoforms. These phosphorylation changes were relatively specific for the chartins and were not observed for phosphorylated MAP 1.2, phospho-beta-tubulin, or most other phosphoproteins. Thus, the phosphorylation state of chartins, but not of other phosphoproteins, is regulated by the presence of native microtubules. Despite depolymerization of microtubules, neurites remained extended for at least 24 h. Neurite elongation, however, was arrested. Microtubules, therefore, may be required for extension, but not for short-term maintenance of well-established neurites. Taxol, which promotes tubule assembly and stability, does not, conversely, drive phosphorylation of the chartins. Instead, taxol appeared to decrease the turnover of phosphate in microtubule-associated, acidic chartin isoforms. These data suggest several models as to how chartin phosphorylation is regulated in neurite-bearing cells and indicate that phosphorylation of cytoplasmic and microtubule-associated chartins occurs via different mechanisms.