Small Ubiquitin-like Modifiers (SUMOs) are a family of small, related proteins that can be enzymatically conjugated to a target protein by a post-translational modification process termed SUMOylation. SUMO-1 is a highly conserved, small ubiquitin-related modifier that has been shown to be covalently conjugated to a large variety of cellular proteins (Kamitani et al., 1997; Mahajan et al., 1997; Matunis et al., 1996). Cloning of SUMO-1 was first described by Boddy et al. (1996). SUMO-2 and SUMO-3 share 95% sequence identity, but only 50% sequence identity to SUMO-1. SUMO-1 is conjugated to a target protein in a similar way to ubiquitin and has been implicated in multiple cellular processes, including nuclear transport, cell cycle control, oncogenesis, inflammation and response to viral infection. SUMO-1 conjugation forms an isopeptide bond between Gly97 at the C-terminus of SUMO-1 and the ε-amino group on the Lysine side chain of the target protein; however it is unable to form multi-chain species (Bayer et al., 1998; Mahajan et al., 1997; Mahajan et al., 1998). SUMO-1 targets substrates including RanGAP1, PML, Sp100, HSF1, Smad4, IκBα, c-Jun, p53 and Mdm2 (Melchior. 2000). RanGAP1, a Ran GTPase-activating is a major SUMO-1 substrate protein involved in nucleocytoplasmic trafficking (Swaminathan et al., 2004). SUMO-1 covalently modifies a single lysine residue at position 526 in the C-terminus of RanGAP1 (Mahajan et al., 1997; Matunis et al., 1996; Muller et al., 1998). SUMO-1 modified RanGAP1 has been found tightly associated with the nuclear envelope (Mahajan et al., 1997; Matunis et al., 1996) an observation which supports its role in nucleocytoplasmic trafficking.
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