Identification of the Proteins, Including MAGEG1, That Make Up the Human SMC5-6 Protein Complex

Abstract

The SMC protein complexes play important roles in chromosome dynamics. The function of the SMC5-6 complex remains unclear, though it is involved in resolution of different DNA structures by recombination. We have now identified and characterized the four non-SMC components of the human complex and in particular demonstrated that the MAGEG1 protein is part of this complex. MAGE proteins play important but as yet undefined roles in carcinogenesis, apoptosis, and brain development. We show that, with the exception of the SUMO ligase hMMS21/ hNSE2, depletion of any of the components results in degradation of all the other components. Depletion also confers sensitivity to methyl methanesulfonate. Several of the components are modified by sumoylation and ubiquitination.SMC (structural maintenance of chromosomes) proteins form the cores of three essential protein complexes involved in important aspects of chromosome dynamics (see reference 15 for a review). Cohesin, containing Smc1 and Smc3, is the molecular glue that holds sister chromatids together from the time that they are established during DNA replication until they separate in anaphase. Smc2 and Smc4 form the core of condensin, which plays a crucial role in compaction of chromosomes at the beginning of mitosis. The function of the SMC5-6 complex is less clear, but it has an important role in a variety of different DNA repair processes and in resolving recombination structures.The SMC proteins are conserved throughout eukaryotes, and all have similar structures. Globular domains at the N and C termini are separated by two long coiled-coil domains linked together by a hinge. Each SMC protein is doubled back at the hinge region such that the coiled-coil domains interact with each other in an antiparallel manner and the globular domains are brought together. In this way a Walker A motif in the N-terminal domain and a Walker B motif in the C-terminal domain are juxtaposed to generate a globular ATP binding and hydrolysis "head" domain. The two Smc proteins in each complex interact with each other via the hinge. In cohesin the head domains of Smc1 and Smc3 are also brought together with the non-Smc components to form a ring, which is thought to encircle the two sister chromatids. It is not known if the other Smc complexes also form such a ring structure.The rad18 gene of Schizosaccharomyces pombe was isolated as a mutant sensitive to both UV and ionizing radiation. It was subsequently found to encode an SMC protein (12) that was redesignated Smc6 as part of the SMC5-6 complex. Further genetic and biochemical studies revealed a role in the repair of double-strand breaks as well as in other repair processes involving repair by homologous recombination (12, 29). The precise function remains unclear. The SMC5-6 complex is essential for cell proliferation, but cells are still able to go through several rounds of division in the absence of Smc6 (9). This suggests that death might result from an accumulation of unrepaired damage. Recent work has shown an accumulation of SMC...

DNA repair, DNA replication and human disorders: A personal journey

“…We discovered its Smc partner protein Smc5 [55], as well as four other members of the Smc5-6 complex, Nse1-4 [56] and worked out how the various components fit together [56,57]. We also have identified and analysed the orthologous complex in human cells [58] and generated mice mutated in Smc6 (unpublished).…”

When MAGE meets RING: insights into biological functions of MAGE proteins

“…Exciting progress in this field was achieved when several groups' data are pointing to RING domain proteins as binding partners for both type I and type II MAGEs (Pebernard et al, 2004;Rual et al, 2005;Sergeant et al, 2005;Yang et al, 2007;Taylor et al, 2008). For example, the interspersed repeat domain of NRAGE/MAGE-D1 (herein referred to as MAGE-D1 for simplicity) was identified to bind RING domain protein XIAP and the avian IAP homolog ITA, members of the inhibitor of apoptosis (IAP) family (Jordan et al, 2001).…”

“…The human SMC5/6 complex comprises six components: SMC5, SMC6, and NSE (Non-SMC Elements) 1-4. MAGE-G1 was identified to be the human ortholog of NSE3 (Taylor et al, 2008;Potts, 2009). Thus, the MAGE-G1-NSE1 complex is special in that they actually represent a sub-complex within the larger SMC5/6 complex.…”

“…The first identified member of this family, MZ2-E, was discovered in a patient with melanoma who had cytotoxic T cell recognizing tumor cells (van der Bruggen et al, 1991;Simpson et al, 2005). This gene was later found to belong to a 12-hMAGE-A genes cluster, and thus known as MAGE-A1 (Chomez et al, 2001).…”

“…Typically, this family is divided into three sub-families: melanoma antigen (MAGE), G antigen (GAGE), and X chromosome antigen (XAGE) families. The majority of these genes exist as multigene families, and are often under similar transcriptional regulation, leading to their co-expression in some contexts (Scanlan et al, 2002).The first identified member of this family, MZ2-E, was discovered in a patient with melanoma who had cytotoxic T cell recognizing tumor cells (van der Bruggen et al, 1991;Simpson et al, 2005). This gene was later found to belong to a 12-hMAGE-A genes cluster, and thus known as MAGE-A1 (Chomez et al, 2001).…”