Family S78


Summary Holotypes Alignment Tree Genomes Literature

Summary for family S78

Family type peptidaseS78.001 - prohead peptidase (bacteriophage HK97) (Enterobacteria phage HK97), MEROPS Accession MER0001396 (peptidase unit: 1-225)
Content of familyPeptidase family S78 contains endopeptidases from bacteriophages.
History 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.
Catalytic typeSerine
Active site residuesH65 S114 
Active siteThe 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 specificitiesIn bacteriophage HK97, the coat protein is processed at a lysyl bond by the prohead endopeptidase.
Molecular structure
Basis of clan assignmentPredicted active site residues for members of this family and family S21 occur in the same order in the sequence: H, D, S.
Distribution of family Bacteria details  
Archaea details  
Protozoa details  
Fungi -  
Plants details  
Animals -  
Viruses details  
Biological functionsIn bacteriophage HK97, the prohead peptidase activates the coat protein allowing maturation of the phage prohead. In bacteriophage Mu, the peptidase degrades the scaffold protein around which the phage prohead is built.
Statistics for family S78Sequences:1517
Identifiers with PDB entries:0
Downloadable files Sequence library (FastA format)
Sequence alignment (FastA format)
Phylogenetic tree (Newick format)
Peptidases and Homologues MEROPS ID Structure
prohead peptidase (bacteriophage HK97)S78.001-
prohead peptidase (bacteriophage T5)S78.002-
family S78 non-peptidase homologuesnon-peptidase homologue-
family S78 unassigned peptidasesunassigned-