XMAP310 - A XENOPUS RESCUE-PROMOTING FACTOR LOCALIZED TO THE MITOTIC SPINDLE

To understand the role of microtubule-associated proteins (MAPs) in th e regulation of microtubule (MT) dynamics we have characterized MAPs p repared from Xenopus laevis eggs (Andersen, S.S.L., B. Buendia, J.E. D ominguez, A. Sawyer, and E. Karsenti. 1994. J. Cell Biol. 127:1289-129 9). Here we report on the purification and characterization of a 310-k D MAP (XMAP310) that localizes to the nucleus in interphase and to mit otic spindle MTs in mitosis. XMAP310 is present in eggs, oocytes, a Xe nopus tissue culture cell line, testis, and brain. We have purified XM AP310 to homogeneity from egg extracts. The purified protein cross-lin ks pure MTs. Analysis of the effect of this protein on MT dynamics by time-lapse video microscopy has shown that it increases the rescue fre quency 5-10-fold and decreases the shrinkage rate twofold. It has no e ffect on the growth rate or the catastrophe frequency. Microsequencing data suggest that XMAP230 and XMAP310 are novel MAPs. Although the th ree Xenopus MAPs characterized so far, XMAP215 (Vasquez, R.J., D.L. Ga rd, and L. Cassimeris. 1994. J. Cell Biol. 127:985-993), XMAP230, and XMAP310 are localized to the mitotic spindle, they have distinct effec ts on MT dynamics. While XMAP215 promotes rapid MT growth, XMAP230 dec reases the catastrophe frequency and XMAP310 increases the rescue freq uency. This may have important implications for the regulation of MT d ynamics during spindle morphogenesis and chromosome segregation.