How Does SUMO Participate in Spindle Organization?

Abstract

The ubiquitin-like protein SUMO is a regulator involved in most cellular mechanisms. Recent studies have discovered new modes of function for this protein. Of particular interest is the ability of SUMO to organize proteins in larger assemblies, as well as the role of SUMO-dependent ubiquitylation in their disassembly. These mechanisms have been largely described in the context of DNA repair, transcriptional regulation, or signaling, while much less is known on how SUMO facilitates organization of microtubule-dependent processes during mitosis. Remarkably however, SUMO has been known for a long time to modify kinetochore proteins, while more recently, extensive proteomic screens have identified a large number of microtubule- and spindle-associated proteins that are SUMOylated. The aim of this review is to focus on the possible role of SUMOylation in organization of the spindle and kinetochore complexes. We summarize mitotic and microtubule/spindle-associated proteins that have been identified as SUMO conjugates and present examples regarding their regulation by SUMO. Moreover, we discuss the possible contribution of SUMOylation in organization of larger protein assemblies on the spindle, as well as the role of SUMO-targeted ubiquitylation in control of kinetochore assembly and function. Finally, we propose future directions regarding the study of SUMOylation in regulation of spindle organization and examine the potential of SUMO and SUMO-mediated degradation as target for antimitotic-based therapies.

Keywords: SUMO; SUMO-targeted ubiquitin ligases; microtubule-associated proteins; mitosis; spindle.

Figure 1

The SUMO pathway [13,18,19]. Similar to ubiquitylation, the SUMO conjugation pathway leads to formation of an isopeptide bond between the C-terminal carboxyl of the 11kDa protein SUMO and an ε-lysine of the target protein. SUMO precursors are first processed by SUMO isopeptidases, leaving a C-terminal di-glycine (GG) motif that is activated in an ATP-dependent manner, forming a thioester with catalytic cysteine of the heterodimeric E1-activating enzyme Uba2/Aos1. Next, SUMO is transferred onto the catalytic cysteine of the E2-conjugating enzyme (Ubc9) and finally onto the target protein, either directly or through a SUMO E3 ligase. SUMO itself can also serve as target for SUMOylation, leading to formation of polySUMO chains. SUMOylation is reversed by one of the SUMO isopeptidases that processed its precursor in the first step (SENP1-3 and SENP5-7 in human and Ulp1 and 2 in budding yeast). SENP1, 2, 3, and 5 belong to the Ulp1 branch, whereas SENP6 and 7 are more closely related to the Smt4/Ulp2 branch [20].

Figure 2

SUMO targets on the mitotic spindle in metaphase (left) and anaphase/telophase (right). In metaphase, SUMO1 (colored in red) mainly localizes around spindle poles and MTs while SUMO2/3 (colored in purple) is enriched at centromeres/kinetochores and around chromosomes. After the metaphase/anaphase transition, SUMO1 relocalizes to the spindle midzone, and SUMO2/3 remains around chromosomes while also localizing to the spindle midzone [36,74]. SUMOylated targets are indicated at the different mitotic locations. Shown are proteins whose SUMOylation has been studied during mitosis.

Figure 3

The kinetochore may contain group SUMOylated subcomplexes. A large number of kinetochore proteins have been identified as SUMO conjugates (see Table 2). Specific subcomplexes may be present as group SUMOylated complexes stabilized through SUMO– SUMO interacting motifs (SIM) interactions between their members. Shown here are the studied cases: The mammalian CENPH/I/K complex, MIS18BP1, CENPE, NUF2, and BUB1B/BuBR1 or the Aurora B-CPC complex (CPC, Survivin-Borealin-INCENP, for references see Table 1), but many more may exist (see §4).

Figure 4

STUbL-dependent ubiquitylation may act as a quality control during kinetochore assembly. RNF4 has been shown to antagonize SENP6 and ubiquitylate the centromeric proteins CENPI, CENPH, and MIS18BP1, factors that are required for loading of CENPA onto centromeres. In addition, yeast Slx5/RNF4 ubiquitylates the centromeric histone Cse4/CENPA for degradation and interacts in yeast 2-hybrid with the inner kinetochore factors Ndc10 and Cep3. STUbLs may also eliminate mislocalized MAPs. Kar9, present normally only on astral microtubule plus-ends, is targeted for degradation by yeast STUbLs when it reaches the plus-ends of kinetochore microtubules. Shown here are the yeast proteins; for references, see text and Table 1.