TROPOMYOSIN IN PREIMPLANTATION MOUSE DEVELOPMENT - IDENTIFICATION, EXPRESSION, AND ORGANIZATION DURING CELL-DIVISION AND POLARIZATION

Tropomyosin is an actin-binding cytoskeletal protein which has been ex tensively characterized in a variety of cell types and tissues, with t he exception of very early developmental stages during which cellular polarization first occurs. We have identified five polypeptides in mou se preimplantation conceptuses which show many of the characteristics of tropomyosin. They form the major portion of the heat-stable cytoske letal protein fraction of blastomeres and have the characteristic isoe lectric and SDS-PAGE migration characteristics on 1-D and 2-D gels. Al l five polypeptides were synthesized in late 2- and 4-cell, and all 8- cell stages, with three of the five polypeptides showing lower synthet ic levels in fertilized eggs and early 2-cell conceptuses. These heat- stable proteins showed specific differences from proteins isolated fro m mouse 3T3 fibroblasts by the same method, namely higher M-r isoforms were not represented, also some of the isoforms can be labeled by inc orporation of [C-14]proline. The cellular distribution of tropomyosin in early stage conceptuses was examined using monoclonal and affinity- purified polyclonal antibodies. Tropomyosin becomes associated both wi th the blastomere cortex postfertilization and with the cleavage furro w during cytokinesis. The interphase cortical association is uniform u ntil the 8-cell stage, when tropomyosin becomes associated with the de veloping apical pole and is excluded from the basolateral cortex. This polar localization is inherited along with the pole at the 8- to 16-c ell division, but experiments in which cell division is artificially p rolonged show that tropomyosin localization does not represent a perma nent marking of the pole. We conclude that the early mouse conceptus c ontains a unique and specific set of tropomyosins which respond to pol arizing signals. (C) 1998 Academic Press.