JCB_201703096 4041..4052

Septins are proteins that assemble into heterooligomeric complexes and form nonpolar filaments that associate with cellular membranes, actin filaments, and microtubules (Hall et al., 2008; Saarikangas and Barral, 2011; Mostowy and Cossart, 2012). Septins act as scaffolds for protein recruitment and as diffusion barriers for subcellular compartmentalization (Caudron and Barral, 2009; Trimble and Grinstein, 2015). They are implicated in numerous biological processes, and their mutation or aberrant expression has been linked to various human pathologies (Mostowy and Cossart, 2012; Dolat et al., 2014; Fung et al., 2014; Angelis and Spiliotis, 2016; Pagliuso et al., 2016). 13 different septin genes have been identified in humans (SEPT1–SEPT12 and SEPT14), which can be classified into four groups based on the encoded protein sequence homology (i.e., the SEPT2 group, the SEPT3 group, the SEPT6 group, and the SEPT7 group; Fig. 1; Kinoshita, 2003; Weirich et al., 2008; Mostowy and Cossart, 2012; Fung et al., 2014). These human genes encode for >30 protein isoforms with tissue-specific expressions. As a result, only a subset of septins may be expressed in a given cell type and the composition of septin filaments may differ in different cells or tissues. All septins contain a central GTP-binding domain flanked by Nand C-terminal domains that vary in length and sequence between different septin groups (Fig. 1 a; Pan et al., 2007; Weirich et al., 2008). Structural studies revealed that the basic unit of septin filaments is composed of the combinatorial association of different septins into hexameric or octameric complexes (Fig. 1 b; Sirajuddin et al., 2007; Kim et al., 2011; Sellin et al., 2011). In humans, these complexes are made up of SEPT7–SEPT6–SEPT2–SEPT2–SEPT6–SEPT7 and/ or SEPT9–SEPT7–SEPT6–SEPT2–SEPT2–SEPT6–SEPT7– SEPT9 alternations, with each member of a group possibly able to substitute for another member of the same group. Filaments formed by the end-on-end assembly of these complexes can pair with one another and form higher-order filamentous structures, such as linear bundles, circular rings, or gauzes. Although the GTP-binding domain and part of the Nand C-terminal regions constitute crucial interfaces for septin–septin interactions, the C-terminal domain of septins, which protrudes orthogonally Septins are cytoskeletal proteins that assemble into nonpolar filaments. They are critical in diverse cellular functions, acting as scaffolds for protein recruitment and as diffusion barriers for subcellular compartmentalization. Human septins are encoded by 13 different genes and are classified into four groups based on sequence homology (SEPT2, SEPT3, SEPT6, and SEPT7 groups). In yeast, septins were among the first proteins reported to be modified by SUMOylation, a ubiquitin-like posttranslational modification. However, whether human septins could be modified by small ubiquitin-like modifiers (SUMOs) and what roles this modification may have in septin function remains unknown. In this study, we first show that septins from all four human septin groups can be covalently modified by SUMOs. We show in particular that endogenous SEPT7 is constitutively SUMOylated during the cell cycle. We then map SUMOylation sites to the C-terminal domain of septins belonging to the SEPT6 and SEPT7 groups and to the N-terminal domain of septins from the SEPT3 group. We finally demonstrate that expression of non-SUMOylatable septin variants from the SEPT6 and SEPT7 groups leads to aberrant septin bundle formation and defects in cytokinesis after furrow ingression. Altogether, our results demonstrate a pivotal role for SUMOylation in septin filament bundling and cell division. SUMOylation of human septins is critical for septin filament bundling and cytokinesis