Less kinesin, more condensation

Actin branches out in yeast any of the same proteins control actin polymerization in budding yeast and mammalian cells. But budding yeast researchers—hampered by an emphasis on genetics and an inaccessible yeast cytoplasm dense with ribosomes and glycogen granules—have often lagged behind in providing structural descriptions based on electron microscopy (EM). This has fueled suspicions about whether the study of yeast actin is relevant to human systems. Now, Young et al. (page 629) get the structural ball rolling with an EM characterization of yeast cortical actin patches, which are the most prominent actin structures in budding yeast. They find significant structural parallels between the yeast and mam-malian systems, suggesting that the smaller yeast structures may be a handy model for tackling problems of actin dynamics. The team partially purified patches from cells expressing GFP-labeled capping protein. Once cells were lysed, the researchers stabilized and cross-linked the actin patches and used correlated fluorescence and electron microscopy to visualize them. Actin filaments were arrayed in a branched fashion, with the branch placement and angles characteristic of Arp2/3 complex– induced branching seen in mammalian cells. The authors conclude that a modified form of the dendritic nucleation model, which is used to explain actin dynamics at the leading edge of motile mammalian cells, does apply to yeast actin dynamics. The relevance of this model to yeast had been questioned because M Less kinesin, more condensation inesin motors drag cargos, including chromosomes, but do not normally reshape those cargos. But on page 613, Mazumdar et al. demonstrate that a human chromo-kinesin HKIF4A is needed to establish the correct condensation state of chromosomes. Chromokinesins are localized along chromosome arms and are thought, at least in some cases, to act as part of the polar wind: they walk along microtubules away from centrosomes, thus dragging their chromosome cargos toward the middle of the mitotic spindle. The authors depleted HKIF4A from human fibroblast cells using antibodies and RNAi. They observed numerous mitotic defects including misaligned chromosomes, incomplete chromosome separation during ana-phase, and disorganized spindles. The K the concentration of actin protein in yeast appeared to be too low to support dendritic nucleation. The yeast actin patches are known to move as they drive endocytosis. In the isolated patches, the branching was equal in all directions, but patches isolated directly from the cortex of living cells (both wild type and mutant) may reveal what process provides directionality. ᭿ Yeast …