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 many cellular protein targets. Proteins modified by SUMO-1 and the SUMO-l-activating 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, to a carrier protein is sufficient for modification in vitro, modification in vivo requires the additional presence of a nuclear localization signal. Thus, protein substrates must be targeted to the nucleus to undergo SUMO-1 conjugation.