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 d escribe a novel protein modification pathway in yeast which mediates t he conjugation of RUB1, a ubiquitin-like protein displaying 53% amino acid identity to ubiquitin. Mire show that RUB1 conjugation requires a t least three proteins in vivo. ULA1 and UBA3 are related to the N-and C-terminal domains of the E1 ubiquitin-activating enzyme, respectivel y, and together fulfil E1-like functions for RUB1 activation. RUB1 con jugation also requires UBC12, a protein related to E2 ubiquitin-conjug ating enzymes, which functions analogously to E2 enzymes in RUB1-prote in in conjugate formation, Conjugation of RUB1 is not essential for no rmal cell growth and appears to be selective fur a small set of substr ates. Remarkably CDC53/cullin, a common subunit of the multifunctional SCF ubiquitin ligase, was found to be a major substrate for RUB1 conj ugation. This suggests that the RUB1 conjugation pathway is functional ly affiliated to the ubiquitin-proteasome system and mag play a regula tory role.