|Family type peptidase||S78.001 - prohead peptidase (bacteriophage HK97) (Enterobacteria phage HK97), MEROPS Accession MER0001396 (peptidase unit: 1-225)|
|Content of family||Peptidase family S78 contains endopeptidases from bacteriophages.|
Identifier created: MEROPS 9.6 (29 February 2012)|
Bacteriophage assembly is a multi-stage procedure, in which the proteins of the head assemble first into the prohead. Usually, a prohead is formed around a central scaffold protein that is subsequently degraded and replaced by the viral DNA. Bacteriophage HK97 is unusual in that a scaffold protein is not involved in prohead assembly. Instead, the prohead is formed from 415 copies of the 42-kDa coat protein which are in an uncleaved, precursor form. Each of these coat protein molecules is processed by removal of a 102-residue N-terminal propeptide. Cleavage allows the 31-kDa coat proteins to cross-react, leading to a structural change that causes head expansion and maturation (Duda et al., 1995). The processing peptidase (S78.001) was identified as the product of the gene adjacent to that of the coat protein (Duda et al., 1995). In bacteriophage Mu, phage maturation is more typical, with a scaffold protein Z involved. The scaffold protein is degraded by a peptidase (S78.002) that is the product of the I gene (Grimaud, 1996); protein Z is a product of the same gene by alternative initiation.
|Active site residues||H65 S114 |
|Active site||The active site residues are not known. However, from computational studies (Liu & Mushegian, 2004, Cheng et al., 2004) it has been suggested that family S78 contains serine peptidases distantly related to family S21 (herpes virus assemblin) and that His65 and Ser114 form a catalytic dyad (see the Alignment). However, in family S21, there is a catalytic triad in which a second His residue takes part; the corresponding residue in S78 is either poorly conserved (Cheng et al., 2004) or is replaced by Glu or Asp (Liu & Mushegian, 2004).|
|Activities and specificities||In bacteriophage HK97, the coat protein is processed at a lysyl bond by the prohead endopeptidase.|
|Basis of clan assignment||Predicted active site residues for members of this family and family S21 occur in the same order in the sequence: H, D, S.|