Identification of Two Type V Myosins in Fission Yeast, One of Which Functions in Polarized Cell Growth and Moves Rapidly in the Cell

We characterized the novel Schizosaccharomyces pombe genes myo4(+) and myo5(+), both of which encode myosin-V heavy chains. Disruption of myo4 caused a defect in cell growth and led to an abnormal accumulation of secretory vesicles throughout the cytoplasm. The mutant cells were rounder than normal, although the sites for cell polarization were still established. Elongation of the cell ends and completion of septation required more time than in wild-type cells, indicating that Myo4 functions in polarized growth both at the cell ends and during septation. Consistent with this conclusion, Myo4 was localized around the growing cell ends, the medial F-actin ring, and the septum as a cluster of dot structures. In living cells, the dots of green fluorescent protein-tagged Myo4 moved rapidly around these regions. The localization and movement of Myo4 were dependent on both F-actin cables and its motor activity but seemed to be independent of microtubules. Moreover, the motor activity of Myo4 was essential for its function. These results suggest that Myo4 is involved in polarized cell growth by moving with a secretory vesicle along the F-actin cables around the sites for polarization. In contrast, the phenotype of myo5 null cells was indistinguishable from that of wild-type cells. This and other data suggest that Myo5 has a role distinct from that of Myo4.

我们对新型裂殖酵母（Schizosaccharomyces pombe）的myo4(+)与myo5(+)基因进行了系统表征，二者均编码肌球蛋白V重链（myosin-V heavy chains）。myo4基因的功能缺失会导致细胞生长缺陷，并引发分泌囊泡（secretory vesicles）在整个细胞质内异常聚集。尽管细胞极化（cell polarization）位点仍可正常形成，但突变体细胞较野生型更为圆润。细胞末端伸长与隔膜形成（septation）的完成所需时间均长于野生型细胞，这表明Myo4在细胞末端极化生长及隔膜形成过程中均发挥功能。与该结论相符的是，Myo4以点状簇结构的形式定位于正在生长的细胞末端、内侧F-肌动蛋白环（F-actin ring）以及隔膜区域。在活体细胞中，经绿色荧光蛋白（green fluorescent protein，GFP）标记的Myo4点状结构可在上述区域快速移动。Myo4的定位与移动依赖于F-肌动蛋白丝束（F-actin cables）及其马达活性（motor activity），但似乎与微管（microtubules）无关。此外，Myo4的马达活性对其功能行使至关重要。上述结果提示，Myo4可通过与分泌囊泡一同沿极化位点周围的F-肌动蛋白丝束移动，参与细胞的极化生长过程。与之相反，myo5基因敲除细胞的表型与野生型细胞无显著差异。结合其他实验数据可知，Myo5的功能与Myo4存在显著差异。