SUMO modification of septin-interacting proteins in Candida albicans

The initiation of bud and hyphal growth in the opportunistic fungal pathogen Candida albicans both involve polarized morphogenesis. However, there are many differences including the function of the septin proteins, a family of proteins involved in membrane organization in a wide range of organisms. Septins form a characteristic ring on the inner surface of the plasma membrane at the bud neck, whereas the septins are diffusely localized across emerging hyphal tips. In addition, septin rings are maintained at sites of septum formation in hyphae rather than being disassembled immediately after cytokinesis. The possibility that C. albicans septins are regulated by the small ubiquitin-like protein SUMO was examined in this study because the Saccharomyces cerevisiae septins were shown previously to be modified by SUMO (Smt3p). However, SUMO conjugation to septins was not detected during budding or hyphal morphogenesis in C. albicans. These results are supported by the lack of conserved SUMO consensus motifs between septins from the two organisms even after adjusting the predicted Cdc3p and Cdc12p septin sequences to account for mRNA splicing in C. albicans. Interestingly, a homolog of the Smt3p SUMO was identified in the C. albicans genome, and an epitope tagged version of Smt3p was conjugated to a variety of proteins. Immunofluorescence analysis showed prominent Smt3p SUMO localization at bud necks and sites of septum formation in hyphae similar to the septins. However, Smt3p was primarily detected on the mother cell side of the septin ring. A subset of these Smt3p-modified proteins co-immunoprecipitated with the septin Cdc11p. These results indicate that septin-associated proteins and not the septins themselves are the key target of SUMO modification at the bud neck in C. albicans.     

SUMO protein modification

SUMO (small ubiquitin-related modifier) family proteins are not only structurally but also mechanistically related to ubiquitin in that they are posttranslationally attached to other proteins. As ubiquitin, SUMO is covalently linked to its substrates via amide (isopeptide) bonds formed between its C-terminal glycine residue and the epsilon-amino group of internal lysine residues. The enzymes involved in the reversible conjugation of SUMO are similar to those mediating the ubiquitin conjugation. Since its discovery in 1996, SUMO has received a high degree of attention because of its intriguing and essential functions, and because its substrates include a variety of biomedically important proteins such as tumor suppressor p53, c-jun, PML and huntingtin. SUMO modification appears to play important roles in diverse processes such as chromosome segregation and cell division, DNA replication and repair, nuclear protein import, protein targeting to and formation of certain subnuclear structures, and the regulation of a variety of processes including the inflammatory response in mammals and the regulation of flowering time in plants.     

小泛素相关修饰物SUMO研究进展

蛋白质翻译后修饰对改变蛋白功能、活性或定位都起着非常重要的作用,泛素及其相似蛋白的修饰是其中一种重要形式。与其他诸如磷酸化、乙酰化、糖基化等不同的是,泛素及其相似蛋白的修饰基团本身即是一个小的多肽,通过异肽键与靶蛋白Lys侧链ε-NH2相连,其中小泛素相关修饰物(small ubiquitin—related modifier,SUMO)与蛋白的共价连接是一种新的广泛存在的翻译后修饰形式。SUMO是广泛存在于真核生物中高度保守的蛋白家族,在脊椎动物中有三个SUMO基因,称为SUMO-1,-2,-3,与泛素在二级结构上极其相似,且催化修饰过程的酶体系也具有很高的同源性。然而,与泛素化介导的蛋白酶降解途径不同,SUMO化修饰发挥着更为广泛的功能,如核质转运、细胞周期调控、信号转导、转录活性调控等。     

Characterization of a novel posttranslational modification in polypyrimidine tract-binding proteins by SUMO1

Polypyrimidine tract-binding protein 1 (PTBP1) and its brainspecific homologue, PTBP2, are associated with pre-mRNAs and influence pre-mRNA processing, as well as mRNA metabolism and transport. They play important roles in neural differentiation and glioma development. In our study, we detected the expression of the two proteins in glioma cells and predicted that they may be sumoylated using SUMOplot analyses. We confirmed that PTBP1 and PTBP2 can be modified by SUMO1 with co-immunoprecipitation experiments using 293ET cells transiently co-expressing SUMO1 and either PTBP1 or PTBP2. We also found that SUMO1 modification of PTBP2 was enhanced by Ubc9 (E2). The mutation of the sumoylation site (Lys137) of PTBP2 markedly inhibited its modification by SUMO1. Interestingly, in T98G glioma cells, the level of sumoylated PTBP2 was reduced compared to that of normal brain cells. Overall, this study shows that PTBP2 is posttranslationally modified by SUMO1. [BMB Reports 2014; 47(4): 233-238]     

SUMO: a history of modification

The small ubiquitin-like modifier (SUMO) is covalently linked to a variety of proteins and is deconjugated by SUMO-specific proteases. A characteristic of SUMO modification is that the biological consequences of conjugation do not appear proportionate to the small fraction of substrate that is modified. SUMO conjugation appears to alter the long-term fate of the modified protein even though the SUMO may be rapidly deconjugated. Thus an unmodified protein with a history of SUMO modification may have different properties from a protein that never has been modified. Here, the diverse effects of SUMO modification are discussed and models proposed to explain SUMO actions.     

SUMO modification of heterogeneous nuclear ribonucleoproteins

Small ubiquitin-related modifiers (SUMOs) are proteins that are posttranslationally conjugated to other cellular proteins, particularly those that localize and function in the nucleus. Enzymes regulating SUMO modification localize in part to nuclear pore complexes (NPCs), indicating that modification of some proteins may occur as they are translocated between the nucleus and the cytoplasm. Substrates that are regulated by SUMO modification at NPCs, however, have not been previously identified. Among the most abundant cargos transported through NPCs are the heterogeneous nuclear ribonucleoproteins (hnRNPs). HnRNPs are involved in various aspects of mRNA biogenesis, including regulation of pre-mRNA splicing and nuclear export. Here, we demonstrate that two subsets of hnRNPs, the hnRNP C and M proteins, are substrates for SUMO modification. We demonstrate that the hnRNP C proteins are modified by SUMO at a single lysine residue, K237, and that SUMO modification at this site decreases their binding to nucleic acids. We also show that Nup358, a SUMO E3 ligase associated with the cytoplasmic fibrils of NPCs, enhances the SUMO modification of the hnRNP C and M proteins. Based on our findings, we propose that SUMO modification of the hnRNP C and M proteins may occur at NPCs and facilitate the nucleocytoplasmic transport of mRNAs.     

Small ubiquitin-like modifier (SUMO) modification of natively unfolded proteins tau and alpha-synuclein

Sumoylation is an important post-translational modification that provides a rapid and reversible means for controlling the activity, subcellular localization, and stability of target proteins. We have examined the covalent attachment of the small ubiquitin-like modifier (SUMO) proteins to tau and alpha-synuclein, two natively unfolded proteins that define several neurodegenerative diseases. Both brain proteins were preferentially modified by SUMO1, as compared with SUMO2 or SUMO3. Tau contains two SUMO consensus sequences, and mutational analyses identified Lys(340) as the major sumoylation site. Although both tau and alpha-synuclein are targets for proteasomal degradation, only tau sumoylation was affected by inhibitors of the proteasome pathway. Tau is a microtubule-associated protein, whose ability to bind and stabilize microtubules is negatively regulated by phosphorylation. Treatment with the phosphatase inhibitor, okadaic acid, or the microtubule depolymerizing drug, colchicine, up-regulated tau sumoylation. This suggests that SUMO modification may preferentially target a free soluble pool of the substrate. These findings revealed a new, possibly regulatory, modification of tau and alpha-synuclein that may also have implications for their pathogenic roles in neurodegenerative diseases.