Although sumoylation regulates a diverse and growing number of recognized biological processes, the molecular mechanisms by which the covalent attachment of the ubiquitin-like protein SUMO can alter the properties of a target protein remain to be established. To address this question, we have used NMR spectroscopy to characterize the complex of mature SUMO-1 with the C-terminal domain of human RanGAP1. Based on amide chemical shift and 15N relaxation measurements, we show that the C terminus of SUMO-1 and the loop containing the consensus sumoylation site in RanGAP1 are both conformationally flexible. Furthermore, the over-all structure and backbone dynamics of each protein remain unchanged upon the covalent linkage of Lys524 in RanGAP1 to the C-terminal Gly97 of SUMO-1. Therefore, SUMO-1 and RanGAP1 behave as "beads-on-a-string," connected by a flexible isopeptide tether. Accordingly, the sumoylation-dependent interaction of RanGAP1 with the nucleoporin RanBP2 may arise through the bipartite recognition of both RanGAP1 and SUMO-1 rather than through a new binding surface induced in either individual protein upon their covalent linkage. We hypothesize that this conformational flexibility may be a general feature contributing to the recognition of ubiquitin-like modified proteins by their downstream effector machineries.