Microtubule-associated protein 4 (MAP4) regulates assembly, protomer-polymer partitioning and synthesis of tubulin in cultured cells

We depleted MAP4, a ubiquitously expressed microtubule (MT)-associated prot ein previously shown to be capable of stabilizing MTs, from HeLa cells by s tably expressing antisense RNA. These HeLa-AS cells, in which the MAP4 leve l was decreased to 33% of the wild-type level, displayed decreased content of total tubulin (65% of the wild-type level). The partitioning of cellular tubulin into protomer and polymer was altered in HeLa-AS cells: polymeric tubulin was decreased to 46% of the level in control cells, while protomeri c tubulin was increased to 226% of the level in control cells. Tubulin prot ein synthesis was decreased, consistent with the tubulin autoregulation mod el, which proposes that tubulin protomer inhibits its own synthesis. Follow ing release from drug-induced depolymerization, MTs in HeLa-AS cells reform ed more slowly, and showed an increased focus on the centrosome, as compare d to control cells. HeLa-AS cells also appeared to be less bipolar in shape and Batter than control cells. Our data suggest that MAP4 regulates assemb ly level of MTs and, perhaps through this mechanism, is involved in control ling spreading and shape of cells.