Family S77


Summary Holotypes Alignment Tree Genomes Literature

Summary for family S77

NamePeptidase family S77 (phage prohead processing peptidase family)
Family type peptidaseS77.001 - prohead peptidase gp21 (bacteriophage T4) (Enterobacteria phage T4), MEROPS Accession MER0001311 (peptidase unit: 1-212)
Content of familyPeptidase family S77 contains the prohead endopeptidase from bacteriophage T4.
History Identifier created: MEROPS 9.6 (29 February 2012)
Bacteriophage T4 is a double-stranded DNA virus that infects Escherichia coli. Following infection, phage DNA is incorporated into the host genome as a prophage. When the prophage is activated, viral proteins and DNA are synthesized by the host. The proteins assemble around a central scaffold protein into a DNA-free prohead. Cleavage of the capsid proteins confers stability, and degradation of the scaffold protein allows phage DNA to enter the prohead. The prohead becomes bound to the bacterial cell membrane, and the mature phage is able to lyse the host cell to escape into the medium. The product of gene 21 has been shown to be the prohead endopeptidase.
Catalytic typeSerine
Active siteNo active site residues have been determined biochemically or by site-directed mutagenesis. However, it has been suggested that the T4 prohead endopeptidase is a serine peptidase with a His, Ser, His catalytic triad (Liu & Mushegian, 2004, Cheng et al., 2004).
Activities and specificitiesTwo capsid proteins, four assembly core proteins and the B1 protein are cleaved (Showe et al., 1976). Cleavage of proteins P22, IPII, IPIII and P68 occur at selected GluAla bonds (Isobe et al., 1976) and protein P23 is cleaved at a GluGly bond (Showe et al., 1976).
InhibitorsThe endopeptidase is unaffected by the standard inhibitors DFP, iodoacetate, EDTA or EGTA (Showe et al., 1976).
Molecular structureNo tertiary structure has been determined. Cheng et al., 2004 have suggested from bioinformatics studies that the T4 prohead endopeptidase has a fold similar to that of assemblin (S21.001). The T4 prohead endopeptidase is synthesized as a precursor which contains three Glu-Gly bonds and two Glu-Ala bonds, so activation is probably autolytic.
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 -  
Archaea -  
Protozoa -  
Fungi -  
Plants details  
Animals details  
Viruses details  
Biological functionsApproximately one hundred copies of the T4 prohead endopeptidase precursor are incorporated within the prohead (Van Driel et al., 1980). The activated endopeptidase is responsible for the processing of several prohead proteins. The polypeptide removed from the N-terminus of the IPIII protein has been shown to act as a targeting signal for incorporation into the prohead (Mullaney & Black, 1996). The scaffold protein, around which the prohead is assembled, is degraded to at least thirty peptides. This allows entry of viral DNA into the prohead.
Statistics for family S77Sequences:34
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 gp21 (bacteriophage T4)S77.001-
family S77 (general)unassigned-