Family S3


Summary Holotypes Alignment Tree Genomes Structure Literature

Summary for family S3

NamePeptidase family S3 (togavirin family)
Family type peptidaseS03.001 - togavirin (Sindbis virus), MEROPS Accession MER0000269 (peptidase unit: 1-264)
Content of familyPeptidase family S3 contains the viral endopeptidase togavirin.
History Identifier created: Biochem.J. 290:205-218 (1993)
Catalytic typeSerine
Active site residuesH141 D163 S215 
Active siteThe active site residues in the Sindbis virus togavirin are His141, Asp147 and Ser215.
Activities and specificitiesThe only known catalytic activity of togavirin from Sindbis virus is cis cleavage of the Trp264Ser bond at its own C-terminus to release it from the polyprotein. This cleavage site is conserved in all members of the Togaviridae known so far. Following cleavage, the C-terminal Trp264 remains in the active site blocking it and preventing further activity.
InhibitorsNo inhibitors are known.
Molecular structureTogavirin was crystallized by Choi et al., 1991 and the tertiary structure revealed a two beta-barrel fold similar to that of chymotrypsin (S01.001). Because of this and because the active site residues of togavirin and chymotrypsin are the same and occur in the same order in the sequence, family S3 is included in clan PA. Togavirin is a much smaller protein than chymotrypsin, lacking a number of surface loops present in chymotrypsin including those equivalent to the P2 and P3 substrate-binding pockets.
Basis of clan assignmentProtein fold of the peptidase unit for members of this family resembles that of chymotrypsin, the type example for clan PA.
Distribution of family Bacteria -  
Archaea -  
Protozoa -  
Fungi -  
Plants -  
Animals -  
Viruses details  
Biological functionsTogavirin (S03.001) is an endopeptidase of the Togaviridae, a family of single-stranded RNA viruses that includes the Alphavirus Sindbis and Semliki viruses, which are human pathogens transmitted by mosquitos that cause encephalitis. The viral genome encodes two polyproteins, P130 containing structural proteins and P270 containing nostructural proteins, both of which contain an endopeptidase. The P270 polyprotein contains a general polyprotein processing endopeptidase (C09.001), whereas the P130 polyprotein contains togavirin, the coat protein. Togavirin is the N-terminal protein in the polyprotein and the only catalytic activity it possesses is its ability to excise itself from the polyprotein. Following release from the polyprotein, togavirin becomes the core protein which forms a coat around the genomic RNA. This nucleocaspid core is surrounded by a lipid membrane through which 80 glycoprotein 'Spike' proteins penetrate.
Statistics for family S3Sequences:62
Identifiers with PDB entries:1
Downloadable files Sequence library (FastA format)
Sequence alignment (FastA format)
Phylogenetic tree (Newick format)
Peptidases and Homologues MEROPS ID Structure
family S3 non-peptidase homologuesnon-peptidase homologue-
family S3 unassigned peptidasesunassigned-