Ooplasmic redistribution in Tubifex eggs with selectively impaired cortical actin cytoskeleton.

Tubifex eggs contain two configurations of actin cytoskeleton (AC): a cortical thin shell of densely packed actin filaments and an endoplasmic loose network of actin filaments. Here we show that microinjection of botulinum C3 exoenzyme (ADP-ribosyltransferase) into Tubifex eggs disrupts the cortical AC, but not the endoplasmic AC. C3 exclusively ADP-ribosylates a 24-kDa protein in the Tubifex egg, suggesting the involvement of this protein in the regulation of the cortical AC. Among the actin-dependent processes that occur during maturation divisions and the first cell cycle, only outward migration of ooplasm (viz., early stage of the first step of ooplasmic segregation) occurs in C3-injected eggs, although migrating ooplasm is organized into patches rather than a continuous layer as observed in intact eggs. This ooplasmic movement is inhibited by cytochalasin D treatments. These results suggest that the endoplasmic AC is able to move ooplasm centrifugally in the absence of the cortical AC. In contrast, poleward movement of subcortical ooplasm along the egg surface (viz., the second step of ooplasmic segregation) is completely abolished in C3-injected eggs, even though the endoplasmic AC is present in the subcortical ooplasm.