Interaction between rat brain microtubule associated proteins (MAPs) and free ribosomes from Xenopus oocyte: a possible mechanism for the in ovo distribution of MAPs.

The binding of microtubule associated proteins (MAPs) to free 80 S ribosomes isolated from Xenopus laevis oocytes inhibits in vitro tubulin assembly (Jessus et al., 1984). The inhibition of tubulin polymerisation was shown to be dependent upon GTP. The dose of GTP needed to induce 50% of the maximal effect was 0.5 mM. Furthermore, the inhibition is enhanced by pretreatment of the ribosomes with ATP-gamma-S, and partially abolished after phosphatase treatment, which strongly suggests that protein phosphorylation regulated the inhibitory effect. When fluorescent purified MAPs are microinjected into Xenopus laevis oocyte, they cap 1 h later the basal nuclear envelope; in contrast, when the fluorescent MAPs-ribosome complex is injected, the fluorescent MAPs remain in the cytoplasm and never reach the region underlying the nuclear envelope.