Pharmacological and genetic evidence for a role of rootlet and phycoplast microtubules in the positioning and assembly of cleavage furrows in Chlamydomonas reinhardtii

In Chlamydomonas reinhardtii, specialized cytoskeletal structures known as rootlet microtubules are present throughout interphase and mitosis. During cytokinesis, an array of microtubules termed the phycoplast is nucleated from rootlet microtubules and forms coincidentally with the cleavage furrow [Johnson and Porter, 1968: J. Cell Biol. 38:403-425; Holmes and Dutcher, 1989: J. Cell Sci. 94:273-285; Gaffel and el-Gammel, 1990: Protoplasma 156:139-148; Schibler and Huang, 1991: J. Cell Biol. 113:605-614]. We have obtained two independent lines of evidence that support the hypothesis that the rootlet and phycoplast microtubules play a direct role in cleavage furrow placement and assembly. First, the destabilization of spindle and phycoplast microtubules by pharmacological agents was accompanied by the aberrant distribution of actin and a failure of' cytokinesis. Second, we characterized mutant strains that failed to complete cytokinesis properly. Actin and myosin were mislocalized to additional rootlet microtubules in the cyt2-1 strain, and this mislocalization was correlated with the presence of additional cleavage furrows. This evidence suggests that microtubules are necessary for the correct positioning and assembly of functional cleavage furrows in C. reinhardtii.