Peptidase family C16 (EC 3.4.22.-) contains the coronaviruses cysteine
endopeptidases involved in viral polyprotein processing [E1]. All
coronaviruses encodes between one and two accessory cysteine proteinases that
recognize and process one or two sites in the amino-terminal half of the
replicase polyprotein during assembly of the viral replication complex. MHV,
HCoV and TGEV encode two accesssory proteinases, called coronavirus
papain-like proteinase 1 and 2 (PL1-PRO and PL2-PRO). IBV and SARS encodes
only one called PL-PRO [1]. Coronaviruses papain-like proteinases 1 and 2 have
restricted specificities, cleaving respectively two and one bond(s)in the
polyprotein. This restricted activity may be due to extended specificity
sites: Arg or Lys at the cleavage site position P5 are required for PL1-PRO
[2], and Phe at the cleavage site position P6 is required for PL2-PRO [3].
PL1-PRO releases p28 and p65 from the N-terminus of the polyprotein; PL2-PRO
cleaves between p210 and p150.
The peptidase family C16 domain is about 260 amino acids in length. This
domain is predicted to have an α-β structural organisation known as the
papain-like fold. It consists of three α-helices and three strands of
antiparallel β-sheet [4]. The active site of the peptidase family C16
domain consists of a catalytic triad of cysteine, histidine, and aspartic acid
residues [1,2,4,5,6]. The nucleophilic Cys occurs in the motif Asn-Cys-Xaa-Yaa
in which Xaa is an aromatic, hydrophobic residue and Yaa is an aliphatic
hydrophobic amino acid. There is little conservation around the general base
His. The aspartic acid plays a significant, although not essential role, in
orienting and/or stabilizing the substrate in the active site [5]. This
peptidase domain also contains Cys residues involved in the formation of a
zinc-binding finger which connects the left and right hand domains of a
papain-like fold, and may be involved in substrate binding or control the
movement of the catalytic domain [4].
Some proteins known to contain a peptidase C16 domain are listed below:
The autocatalytic release of a putative RNA virus transcription factor from its polyprotein precursor involves two paralogous papain- like proteases that cleave the same peptide bond.
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