A role for mitotic SUMOylation in establishing nuclear envelope structure

Abstract

The compartmentalization of subcellular functions facilitates the regulation of biochemical reactions and cellular processes. The compartmentalization of the eukaryotic genome into the nucleus by the nuclear envelope (NE), for example, facilitates various DNA metabolic activities. The NE is composed of two lipid bilayers: an outer nuclear membrane (ONM) and an inner nuclear membrane (INM). The INM provides an environment for the proper regulation and organization of interacting chromatin. In Saccharomyces cerevisiae, INM-associated chromatin includes telomeres and specific transcriptionally active genes. Multiple mechanisms are employed to tether these chromatin regions to the INM, including post-translational modifications. SUMOylation is a post-translational modification linked to regulating the spatial organization of chromatin relative to the nuclear periphery. Therefore, we investigated the contributions of SUMOylation in mediating chromatin interactions with the INM in response to specific cellular events. We observed that activation of an inducible gene, INO1, is accompanied by alterations in the SUMOylation of proteins associated with specific regions along the INO1 locus. Furthermore, we show that the E3 SUMO ligase, Siz2, is required to facilitate these SUMOylation events and target the INO1 locus to the INM. Following these analyses, we further investigated Siz2 and Siz2-mediated SUMOylation events at the INM. We found that Siz2 is predominantly distributed throughout the nucleoplasm during interphase but is recruited to the INM during mitosis, where it binds and SUMOylates several proteins, including Scs2. Scs2 is an integral iii membrane protein found throughout the endoplasmic reticulum (ER), and by analyses here, the INM. We show that a putative FFAT motif in Siz2 is required to interact with the MSP domain of Scs2. These interactions are further supported by the mitotic phosphorylation of Siz2 and the SUMOylation of Scs2 by Siz2. Formation of the Scs2-Siz2 complex at the INM during mitosis drives the accumulation of SUMO conjugates at the INM, including SUMOylated Scs2 and other specific proteins. The mitotic SUMOylation of these proteins supports the assembly and anchorage of subtelomeric chromatin and the activated INO1 at the INM during the later stages of mitosis and the subsequent G1-phase. The mitotic SUMOylation of these specific proteins is also required for the proliferation of the NE. These SUMOylation events facilitate the accumulation of phosphatic acid (PA) at the INM during mitosis by altering specific protein interactions of PA metabolism regulators. In summary, we have uncovered previously undefined spatial and temporally regulated SUMOylation events mediated by Siz2 at the NE during mitosis. These events function to support and coordinate multiple processes necessary for establishing nuclear envelope structure, including chromatin association with the NE and membrane proliferation.