SUMO-1 conjugation in vivo requires both a consensus modification motif and nuclear targeting

SUMO-1 is a small ubiquitin-related modifier that is covalently linked to m any cellular protein targets. Proteins modified by SUMO-1 and the SUMO-l-ac tivating and -conjugating enzymes are located predominantly in the nucleus. Here we define a transferable sequence containing the psi KXE motif, where psi represents a large hydrophobic amino acid, that confers the ability to be SUMO-l-modified on proteins to which it is linked. Whereas addition of short sequences from p53 and I kappaB alpha containing the psi KXE motif, t o a carrier protein is sufficient for modification in vitro, modification i n vivo requires the additional presence of a nuclear localization signal. T hus, protein substrates must be targeted to the nucleus to undergo SUMO-1 c onjugation.