UBC is the abbreviation of ubiquitin-conjugating. Most ubiquitin-conjugating enzymes (E2s) possess a core UBC domain, which contains the catalytic cysteine residue and interacts with E1s. E2 catalyzes the second step in the ubiquitination reaction, which targets a protein for degradation via the proteasome (1). Besides ubiquitination, E2s catalyzes ubiquitin-like modification. Once conjugated to ubiquitin or ubiquitin-like by E1 enzyme, the E2 enzyme binds one of several ubiquitin ligases or E3s via a structurally conserved binding region. Finally, the target protein substrate gets more and more ubiquitin molecules or ubiquitin-like molecules from E2 by E3.

1. http://en.wikipedia.org/wiki/Ubiquitin-conjugating_enzyme