Regulation of microtubule organization during interphase and M phase

Microtubule (MT) dynamics and organization change markedly during interphas e-hl phase transition of the cell cycle. This mini review focuses first on p220, a ubiquitous MT-associated protein of Xenopus. p220 is phosphorylated by p34(cdc2) kinase and MAP kinase in M phase, and concomitantly loses its MT-binding and MT-stabilizing activities. A cDNA encoding p220 was cloned, which identified p220 as a Xenopus homolog of MAP4, and p220 was therefore termed XMAP4. To examine the physiological relevance of XMAP4 phosphorylat ion during mitosis, Xenopus A6 cells were transfected with cDNA encoding wi ld-type or various XMAP4 mutants fused with a green fluorescent protein (GF P). Mutations of serine and threonine within potential phosphorylation site s for p34(cdc2) kinase to nonphosphorylatable alanine interfered with mitos is-associated reduction in MT-affinity of XMAP4 and their overexpression af fected chromosome movement during anaphase A. These results indicated that phosphorylation of XMAP4 by p34(cdc2) kinase is responsible for the decreas e in its MT-binding and MT-stabilizing activities during mitosis which are important for chromosome movement during anaphase A. The second focus is on a novel monoclonal antibody W8C3, which recognizes a-tubulin. W8C3 stained spindle MTs but not interphase MTs of Xenopus A6 cells, although tubulin d imers in M phase and interphase were equally recognized by this antibody. T he difference in MT staining pattern may be because the W8C3-recognition si te on alpha-tubulin is sterically hidden in interphase MTs but not in spind le MTs.