Evidence for the involvement of microtubules, ER, and kinesin in the cortical rotation of fertilized frog eggs

During the first cell cycle, the vegetal cortex of the fertilized frog egg is translocated over the cytoplasm. This process of cortical rotation creates regional cytoplasmic differences important in later development, and appears to involve an array of aligned microtubules that forms transiently beneath the vegetal cortex. We have investigated how these microtubules might be involved in generating movement by analyzing isolated cortices and sections of Xenopus laevis and Rana pipiens eggs. First, the polarity of the cortical microtubules was determined using the "hook" assay. Almost all microtubules had their plus ends pointing in the direction of cortical rotation. Secondly, the association of microtubules with other cytoplasmic elements was examined. Immunofluorescence revealed that cytokeratin filaments coalign with the microtubules. The timing of their appearance and their position on the cytoplasmic side of the microtubules suggested that they are not involved directly in generating movement. ER was visualized with the dye DiIC16(3) and by immunofluorescence with anti- BiP (Bole, D. G., L. M. Hendershot, and J. F. Kearney, 1986. J. Cell Biol. 102:1558-1566). One layer of ER was found closely underlying the plasma membrane at all times. An additional, deeper layer formed in association with the microtubules of the array. Antibodies to sea urchin kinesin (Ingold, A. L., S. A. Cohn, and J. M. Scholey. 1988. J. Cell Biol. 107:2657-2667) detected antigens associated with both the ER and microtubules. On immunoblots they recognized microtubule associated polypeptide(s) of approximately 115 kD from Xenopus eggs. These observations are consistent with a role for kinesin in creating movement between the microtubules and ER, which leads in turn to the cortical rotation.