A Septin Double Ring Controls the Spatiotemporal Organization of the ESCRT Machinery in Cytokinetic Abscission

Abscission is the terminal step of mitosis that physically separates two daughter cells. Abscission requires the endocytic sorting complex required for transport (ESCRT), a molecular machinery of multiple subcomplexes (ESCRT-I/II/III) that promotes membrane remodeling and scission. Recruitment of ESCRT-I/II complexes to the midbody of telophase cells initiates ESCRT-III assembly into two rings, which subsequently expand into helices and spirals that narrow down to the incipient site of abscission. ESCRT-III assembly is highly dynamic and spatiotemporally ordered, but the underlying mechanisms are poorly understood. Here, we report that after cleavage furrow closure, septins form a membrane-bound double ring that controls the organization and function of ESCRT-III. Flanking the ESCRT-I protein TSG101, the septin double ring demarcates the sites of ESCRT-III assembly into rings and disassembles promptly before ESCRT-III rings expand into helices/spirals. We show that septin 9 (SEPT9) depletion, which abrogates abscission, impairs recruitment of VPS25 (ESCRT-II) and CHMP6 (ESCRT-III). Strikingly, ESCRT-III subunits (CHMP4B/CHMP2A) accumulate to the midbody, but they are highly disorganized failing to form symmetric rings and to expand laterally into the cone-shaped helices/spirals of abscission. We found that SEPT9 interacts directly with the ubiquitin E2 variant (UEV) domain of TSG101 through two N-terminal PTAP motifs, which are required for the recruitment of VPS25 and CHMP6, and the spatial organization of ESCRT-III (CHMP4B) into functional rings. These results reveal that septins function in the ESCRT-IESCRT- II-CHMP6 pathway of ESCRT-III assembly and provide a framework for the spatiotemporal control of the ESCRT machinery of cytokinetic abscission.