Ubc9p and the conjugation of SUMO-1 to RanGAP1 and RanBP2

SUMO-1 modification of IkappaBalpha inhibits NF-kappaB activation

Activation of NF-kappaB is achieved by ubiquitination and proteasome-mediated degradation of IkappaBalpha. We have detected modified IkappaBalpha, conjugated to the small ubiquitin-like protein SUMO-1, which is resistant to signal-induced degradation. In the presence of an E1 SUMO-1-activating enzyme, Ubch9 conjugated SUMO-1 to IkappaBalpha primarily on K21, which is also utilized for ubiquitin modification. Thus, SUMO-1-modified IkappaBalpha cannot be ubiquitinated and is resistant to proteasome-mediated degradation. As a result, overexpression of SUMO-1 inhibits signal-induced activation of NF-kappaB-dependent transcription. Unlike ubiquitin modification, which requires phosphorylation of S32 and S36, SUMO-1 modification of IkappaBalpha is inhibited by phosphorylation. Thus, while ubiquitination targets proteins for rapid degradation, SUMO-1 modification acts antagonistically to generate proteins resistant to degradation.

A novel protein modification pathway related to the ubiquitin system

Ubiquitin conjugation is known to target protein substrates primarily to degradation by the proteasome or via the endocytic route. Here we describe a novel protein modification pathway in yeast which mediates the conjugation of RUB1, a ubiquitin-like protein displaying 53% amino acid identity to ubiquitin. We show that RUB1 conjugation requires at least three proteins in vivo. ULA1 and UBA3 are related to the N- and C-terminal domains of the E1 ubiquitin-activating enzyme, respectively, and together fulfil E1-like functions for RUB1 activation. RUB1 conjugation also requires UBC12, a protein related to E2 ubiquitin-conjugating enzymes, which functions analogously to E2 enzymes in RUB1-protein conjugate formation. Conjugation of RUB1 is not essential for normal cell growth and appears to be selective for a small set of substrates. Remarkably, CDC53/cullin, a common subunit of the multifunctional SCF ubiquitin ligase, was found to be a major substrate for RUB1 conjugation. This suggests that the RUB1 conjugation pathway is functionally affiliated to the ubiquitin-proteasome system and may play a regulatory role.