Involved in protein ubiquitination, but has no ubiquitin ligase activity on its own (PubMed:15530417). The uev-1-ubc-13 heterodimers catalyze the synthesis of non-canonical 'Lys-63'-linked polyubiquitin chains (PubMed:15530417, PubMed:24595290). Involved in sorting Lys-63-linked polyubiquitinated maternal membrane proteins for degradation by targeting to multivesicular bodies (PubMed:24595290). May be involved in the ubiquitination and growth of intracellular polyglutamine protein aggregates (PubMed:17663792, PubMed:22494772). May have a role in AMPA-type glutamate receptor trafficking in neurons (PubMed:21179194). S-ubiquitinyl-[E1 ubiquitin-activating enzyme]-L-cysteine + [E2 ubiquitin-conjugating enzyme]-L-cysteine = [E1 ubiquitin-activating enzyme]-L-cysteine + S-ubiquitinyl-[E2 ubiquitin-conjugating enzyme]-L-cysteine. Protein modification; protein ubiquitination. Interacts with the E3 ubiquitin ligase nhl-1 and the E2 ubiquitin ligase ubc-1 (PubMed:15530417). Heterodimer with uev-1 (PubMed:15530417, PubMed:21179194). Temperature sensitive with partial embryonic lethality at 25 degrees Celsius (PubMed:24595290). Inhibited degradation of maternal membrane proteins, cav-1, chs-1 and rme-2, and ubiquitination of cav-1 with accumulation of these proteins on the maternal plasma membrane and endosome-like vesicles in later-stage embryos (PubMed:24595290). RNAi-mediated knockdown prevents localization of ubiquitin and proteasomes to polyglutamine protein aggregates (PubMed:17663792). Reduced growth of polyglutamine protein aggregates (PubMed:17663792, PubMed:22494772). Inhibited degradation of maternal membrane protein, cav-1 in embryos (PubMed:24595290). Belongs to the ubiquitin-conjugating enzyme family. Has no ubiquitin ligase activity on its own; may require ubiquitin-conjugating enzyme, ubc-13.