BACKGROUND: Ubiquitination plays a critical role in regulating many cellular
processes, from DNA repair and gene transcription to cell cycle and apoptosis.
It is catalyzed by a specific enzymatic cascade ultimately leading to the
conjugation of ubiquitin to lysine residues of the target protein that can be
the ubiquitin molecule itself and to the formation of poly-ubiquitin chains.
FINDINGS: We present the crystal structure at 3.0 A resolution of bovine
ubiquitin crystallized in presence of cadmium ions. Two molecules of ubiquitin
are present in the asymmetric unit. Interestingly this non-covalent dimeric
arrangement brings Lys-6 and Lys-63 of each crystallographically-independent
monomer in close contact with the C-terminal ends of the other monomer. Residues
Leu-8, Ile-44 and Val-70 that form a hydrophobic patch at the surface of the Ub
monomer are trapped at the dimer interface. CONCLUSIONS: The structural basis
for signalling by poly-Ub chains relies on a visualization of conformations of
alternatively linked poly-Ub chains. This arrangement of ubiquitin could
illustrate how linkages involving Lys-6 or Lys-63 of ubiquitin are produced in
the cell. It also details how ubiquitin molecules can specifically chelate
cadmium ions.
