Family S21


Summary Holotypes Alignment Tree Genomes Structure Literature

Summary for family S21

NamePeptidase family S21 (assemblin family)
Family type peptidaseS21.002 - cytomegalovirus assemblin (human herpesvirus 5), MEROPS Accession MER0000613 (peptidase unit: 1-256)
Content of familyPeptidase family S21 contains assemblins, which are processing endopeptidases of the herpesviruses, one of the groups of DNA viruses.
History Identifier created: Biochem.J. 290:205-218 (1993)
Catalytic typeSerine
Active site residuesH63 S132 H157 
Active siteThe catalytic residues form a triad: His, Ser, His (see the Alignment). This is a combination not seen in peptidases of any other family.
Activities and specificitiesA common motif of assemblin cleavage sites is Val/Leu-Xaa-AlaSer, where Xaa is a polar residue. Three sites matching this motif are cleaved by human cytomegalovirus assemblin. The active form of the enzyme is a homodimer, and conditions that alter the degree of dimerisation affect activity. Reducing conditions tend to be necessary for full activity.
InhibitorsDFP is an inhibitor, but other standard inhibitors of serine peptidases have low activity. Synthetic inhibitors have been designed as potential drugs (e.g. Borthwick et al., 2003).
Molecular structureThe tertiary structures determined for assemblins from human cytomegalovirus (S21.002) and the varicella-zoster virus (S21.005) show no resemblance to those of any other peptidase, so family S21 is the single family in its clan, SH. Assemblin does not possess a bilobed structure, but is composed of a single domain and is active as a dimer.
Basis of clan assignmentType family of clan SH.
Distribution of family Bacteria -  
Archaea -  
Protozoa -  
Fungi -  
Plants -  
Animals -  
Viruses details  
Biological functionsAssemblin is involved in the late stages of assembly of the viral prohead, breaking down the scaffold protein upon which the prohead is assembled.
Pharmaceutical and biotech relevanceBecause of their role in the biology of several viruses that cause human disease, the assemblins are drug targets.
Statistics for family S21Sequences:73
Identifiers with PDB entries:5
Downloadable files Sequence library (FastA format)
Sequence alignment (FastA format)
Phylogenetic tree (Newick format)
Peptidases and Homologues MEROPS ID Structure
cytomegalovirus assemblinS21.002Yes
Epstein-Barr virus-type assemblinS21.003Yes
herpesvirus 6-type assemblinS21.004-
Varicella zoster assemblinS21.005Yes
herpesvirus 8-type assemblinS21.006Yes
family S21 non-peptidase homologuesnon-peptidase homologue-
family S21 unassigned peptidasesunassigned-