RING-domain E3 ligases enhance transfer of ubiquitin to substrate proteins by
stabilizing the RING-bound thioester-linked E2∼ubiquitin conjugate in a
defined conformation that primes the active site for nucleophilic attack. Here
we report that the monomeric RING domains from the human E3 ligases Arkadia and
Ark2C bind directly to free ubiquitin with an affinity comparable to that of
other dedicated ubiquitin-binding domains. Further work showed that the Ark-like
RING domain and the noncovalently bound ubiquitin molecule coordinately
stabilize the E2-conjugated ubiquitin (donor ubiquitin) in the 'closed'
conformation. Our studies identify the RING domain of Arkadia as a
ubiquitin-binding domain and provide insight into a new ubiquitin-dependent
mechanism used by monomeric RING domains to activate ubiquitin transfer. This
study also suggests how substrates that have been monoubiquitinated could be
favored for further ubiquitination.
