We have identified a possible role for the KIFC1 motor protein in formation of the acrosome, an organelle unique to spermatogenesis. KIFC1, a C-terminal kinesin motor, first appears on membrane-bounded organelles (MBOs) in the medulla of early spermatids followed by localization to the acrosomal vesicle. KIFC1 continues to be present on the acrosome of elongating spermatids as it flattens on the spermatid nucleus; however, increasing amounts of KIFC1 are found at the caudal aspect of the spermatid head and in distal cytoplasm. The KIFC1 motor is also found in the nucleus of very immature round spermatids just prior to its appearance on the acrosome. In some cases, KIFC1 appears localized just below the nuclear membrane adjacent to the subacrosomal membrane. We demonstrate that KIFC1 is associated with importin β and colocalizes with this nuclear transport factor on curvilinear structures associated with the spermatid nuclei. These data support a model in which KIFC1, perhaps in association with nuclear factors, assists in the formation and/or elongation of the spermatid acrosome. This article represents the first demonstration of a direct association of a molecular motor with the spermatid acrosome, the formation of which is essential for fertilization.
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