报告题目：Cell cycle regulation of actin cable dynamics
主要内容：Assembly of appropriately oriented actin cables nucleated by formin proteins is necessary for many biological processes in diverse eukaryotes. However, compared to knowledge of how nucleation of dendritic actin filament arrays by the Arp2/3 complex is regulated, the in vivo regulatory mechanisms for actin cable formation are less clear. To gain insights into mechanisms for regulating actin cable assembly, we reconstituted the assembly process in vitro by introducing microspheres functionalized with the C-terminus of the budding yeast formin Bni1 into extracts prepared from yeast cells at different cell cycle stages. Electron microscopy studies showed that unbranched actin filament bundles were successfully reconstituted in the yeast extracts. Only extracts enriched in the mitotic cyclin Clb2 were competent for actin cable assembly, and cyclin dependent kinase 1 (Cdk1) activity was indispensible. Cdk1 activity was also found to regulate cable assembly in vivo. Evidence that cell cycle regulation of actin cable assembly is conserved in vertebrates is presented. The cable reconstitution system was used to test roles for the key actin-binding proteins tropomyosin, capping protein and cofilin, providing new insights into assembly regulation. Furthermore, using mass spectrometry, we identified components of the actin cables formed in yeast extracts, providing the basis for comprehensive understanding of cable assembly and regulation. Among these actin cable components we have identified several Cdk1 targets.