Evidence for a ribonucleoprotein complex as a template for microtubule initiation in vivo.

At the onset of mitosis in cultured mammalian cells, the centriolar region rapidly initiates the assembly of microtubules (MT) to form two asters which ultimately forms the basis of the mitotic spindle. The rapid change in the centrosphere's ability to nucleate MTs at prometaphase maybe due to the presence of an RNase sensitive component. Lysis of colcemid-blocked mitotic cells with RNase A or T2 prior to addition of exogenous microtubule protein greatly diminishes the number of MT that can be nucleated in a lysed cell system. MT initiation can also be reduced or abolished by extended lysis in neutral buffers or brief lysis in acidic buffers. This suggests that the RNA component maybe complexed with a protein to serve as a template for MT initiation at the onset of mitosis.