Molecular function

Isoenergetic transfer of SUMO from one protein to an existing ubiquitin chain via the reaction X-ubiquitin + Y-ubiquitin = Y-ubiquitin-ubiquitin + X, where both the X-ubiquitin and Y-ubiquitin-ubiquitin linkages are thioester bonds between the C-terminal glycine of ubiquitin and a sulfhydryl side group of a cysteine residue.