Emerging roles of the SUMO pathway in mitosis

Abstract

SUMO proteins are small ubiquitin-like modifiers found in all eukaryotes that become covalently conjugated to other cellular proteins. The SUMO conjugation pathway is biochemically similar to ubiquitin conjugation, although the enzymes within the pathway act exclusively on SUMO proteins. This post-translational modification controls many processes. Here, I will focus on evidence that SUMOylation plays a critical role(s) in mitosis: Early studies showed a genetic requirement for SUMO pathway components in the process of cell division, while later findings implicated SUMOylation in the control of mitotic chromosome structure, cell cycle progression, kinetochore function and cytokinesis. Recent insights into the targets of SUMOylation are likely to be extremely helpful in understanding each of these aspects. Finally, growing evidence suggests that SUMOylation is a downstream target of regulation through Ran, a small GTPase with important functions in both interphase nuclear trafficking and mitotic spindle assembly.

Figure 1

SUMO pathway. SUMO proteins undergo post-translational maturation, catalyzed by Ulp/SENPs, to reveal a C-terminal di-glycine motif (Step 1). Mature SUMOs undergo ATP-dependent activation, resulting in a thiolester linkage between the C-terminal di-glycine and their activating enzyme, Uba2/Aos1 (Step 2). The thiolester is transferred to their conjugating enzyme, Ubc9 (Step 3). Ubc9 acts in concert with SUMO ligases/E3 enzymes to form an isopeptide linkage between the SUMO C-terminus and an ε-amino group of a lysine within the target protein (Step 4). SUMOs can be removed from conjugated species by the action of Ulp/SENPs (Step 5). In some cases, SUMO chains can be formed through linkage of additional SUMO moieties to previously conjugated SUMOs (Step 6). While it is possible that multiple Ulp/SENPs may disassemble SUMO chains (Step 7), members of the Ulp2 family appear to be specialized for this reaction.

Figure 2

Mitotic chromosomal SUMOylation substrates. The distribution of SUMOylation substrates is schematically represented, based on the localization of the vertebrate homologues. (The localization reflects the bulk of each protein on mitotic vertebrate chromosomes, not specifically the SUMOylated forms.) The names of vertebrate proteins are indicated in black, while corresponding budding yeast proteins are given in red. In cases where SUMOylation has not been confirmed in both vertebrates and yeast, parentheses indicate the homologue for which demonstration is lacking. This representation does not include many proteins identified in proteomic screens [1-4] whose conjugation has not been independently verified, nor yeast proteins without obvious vertebrate homologues (e.g., Ndc10p, Cep3p [63]). Confirmed SUMOylation substrates associated to mitotic chromosomes (e.g., Histones [95]) are similarly not represented if the timing of their modification has not been demonstrated within the cell cycle.