The physical interactions between p37^env-mos and tubulin structures

The c-mos protein has been reported to be complexed with tubulin and to co-localize with microtubules in unfertilized Xenopus eggs as well as in NIH3T3 cells transformed by the Xenopus c-mos gene. We performed experiments to determine whether the viral mos protein, p37^v-mos, also associates with tubulin. Both mouse c-mos and v-mos proteins synthesized in vitro co-polymerized with tubulin. Upon incubation at 37°C, essentially all of the mos protein (both viral and cellular) co-polymerized with tubulin, while more than 50% of the tubulin remained in the depolymerized state. The mos-tubulin interaction was specific, as indicated by the insolubility of the v-mos protein following a second cycle of temperature-dependent depolymerization/polymerization. Beta-tubulin was shown to co-precipitate with p3T^v-mos and to be phosphorylated by the mos kinase in vitro. Although both v-mos and c-mos proteins co-polymerize with tubulin, p37^v-mos behaved differently from p39^c-mos on gel filtration columns under conditions that favor disassembly of microtubules. Like Xenopus c-mos, the bulk of the mouse c-mos protein synthesized in vitro appeared in structures that fractionate at about 500 kDa. In contrast to c-mos, the majority of the v-mos protein, either isolated from stably transformed NIH3T3 cells or synthesized in vitro, eluted in the 100 kDa fraction, cofractionating with tubulin dimers. Therefore, the v-mos protein appears to have a higher affinity for unpolymerized tubulin than c-mos, under conditions that favor disassembly of microtubules.