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13  structures 150  species 3  interactions 270  sequences 1  architecture

Family: SSI (PF00720)

Summary: Subtilisin inhibitor-like

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This is the Wikipedia entry entitled "SSI protease inhibitor". More...

SSI protease inhibitor Edit Wikipedia article

PDB 5sic EBI.jpg
Molecular recognition at the active site of subtilisin BPN': crystallographic studies using genetically engineered protease inhibitor SSI (Streptomyces subtilisin inhibitor)
Symbol SSI
Pfam PF00720
InterPro IPR000691
SCOP 2sic

In molecular biology the protein SSI is a Subtilisin inhibitor-like which stands for Streptomyces subtilisin inhibitor. This is a protease inhibitor. These are often synthesised as part of a larger precursor protein, either as a prepropeptide. The function of this protein domain is to prevent access of the substrate to the active site. It is found only in bacteria.


SSI is a protease inhibitor, it prevents enzymes from acting on a substrate. Some SSI's also inhibit trypsin, chymotrypsin and griselysin.[1][2] Commercially, SSI's have huge potential in the commercial market, they help stabilise proteases in products such as laundry detergents to prevent autolysis of biological washing powders.[3] This means that the enzymes in the washing powder are kept in optimum performance.


SSI is a homodimer, in other words, it is made of two subunits which are exactly the same as each other. Each monomer contains 2 antiparallel beta-sheets and 2 short alpha-helices. Protease binding induces the widening of a channel-like structure, in which hydrophobic side-chains are sandwiched between 2 lobes.[4]

Studies have shown that the loss of the C-terminal domain reduces the inhibitory effect of the proteins. This implies that the C-terminal domain is responsible for maintaining the correct 3D fold.[5]

Structural similarities between the primary and secondary contact loops of SSI, and the ovomucoid and pancreatic secretory trypsin inhibitor family suggest evolution of the 2 families from a common ancestor.[4]


  1. ^ Rawlings ND, Tolle DP, Barrett AJ (March 2004). "Evolutionary families of peptidase inhibitors". Biochem. J. 378 (Pt 3): 705–16. doi:10.1042/BJ20031825. PMC 1224039. PMID 14705960. 
  2. ^ Kojima S, Nishiyama Y, Kumagai I, Miura K (March 1991). "Inhibition of subtilisin BPN' by reaction site P1 mutants of Streptomyces subtilisin inhibitor". J. Biochem. 109 (3): 377–82. PMID 1908859. 
  3. ^ Ganz PJ, Bauer MD, Sun Y, Fieno AM, Grant RA, Correa PE; et al. (2004). "Stabilized variant of Streptomyces subtilisin inhibitor and its use in stabilizing subtilisin BPN'.". Protein Eng Des Sel 17 (4): 333–9. doi:10.1093/protein/gzh045. PMID 15187224. 
  4. ^ a b Hirono S, Akagawa H, Mitsui Y, Iitaka Y (September 1984). "Crystal structure at 2.6 A resolution of the complex of subtilisin BPN' with streptomyces subtilisin inhibitor". J. Mol. Biol. 178 (2): 389–414. doi:10.1016/0022-2836(84)90150-5. PMID 6387152. 
  5. ^ Sakai M, Odani S, Ikenaka T (March 1980). "Importance of the carboxyl-terminal four amino acid residues in the inhibitory activity of Streptomyces subtilisin inhibitor (with a revision of its carboxyl-terminal sequence)". J. Biochem. 87 (3): 891–8. PMID 6993452. 

This article incorporates text from the public domain Pfam and InterPro IPR000691

This page is based on a Wikipedia article. The text is available under the Creative Commons Attribution/Share-Alike License.

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Subtilisin inhibitor-like Provide feedback

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This tab holds annotation information from the InterPro database.

InterPro entry IPR023549

The Streptomyces family of bacteria produce a number of proteinase inhibitors, which belong to MEROPS inhibitor family I16, clan IY. They are characterised by their strong activity towards subtilisin (MEROPS peptidase family S8, INTERPRO) and are collectively known as Streptomyces subtilisin inhibitors (SSI). Some SSI also inhibit trypsin, chymotrypsin (MEROPS peptidase family S1, INTERPRO) and griselysin (MEROPS peptidase family M4, INTERPRO) [PUBMED:14705960]. Mutation of the active site residue can influence inhibition specificity [PUBMED:1908859].

SSI is a homodimer, each monomer containing 2 anti-parallel beta-sheets and 2 short alpha-helices. Protease binding induces the widening of a channel-like structure, in which hydrophobic side-chains are sandwiched between 2 lobes [PUBMED:6387152]. Loss of the C-terminal tetrapeptide VFAF drastically reduces the inhibitory effect of the proteins when there is less than one molecule of inhibitor present per molecule of enzyme. This implies that the tetrapeptide is neccessary to maintain the correct 3D fold [PUBMED:6993452]. Structural similarities between the primary and secondary contact loops of SSI, and the ovomucoid and pancreatic secretory trypsin inhibitor family suggest evolution of the 2 families from a common ancestor [PUBMED:6387152].

Gene Ontology

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Domain organisation

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Seed source: Pfam-B_679 (release 2.1)
Previous IDs: none
Type: Domain
Author: Bateman A
Number in seed: 25
Number in full: 270
Average length of the domain: 88.90 aa
Average identity of full alignment: 34 %
Average coverage of the sequence by the domain: 61.55 %

HMM information View help on HMM parameters

HMM build commands:
build method: hmmbuild -o /dev/null HMM SEED
search method: hmmsearch -Z 80369284 -E 1000 --cpu 4 HMM pfamseq
Model details:
Parameter Sequence Domain
Gathering cut-off 21.8 21.8
Trusted cut-off 21.8 23.4
Noise cut-off 21.1 20.6
Model length: 92
Family (HMM) version: 13
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Species distribution

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Archea Archea Eukaryota Eukaryota
Bacteria Bacteria Other sequences Other sequences
Viruses Viruses Unclassified Unclassified
Viroids Viroids Unclassified sequence Unclassified sequence


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There are 3 interactions for this family. More...

Trypsin Peptidase_M7 Peptidase_S8


For those sequences which have a structure in the Protein DataBank, we use the mapping between UniProt, PDB and Pfam coordinate systems from the PDBe group, to allow us to map Pfam domains onto UniProt sequences and three-dimensional protein structures. The table below shows the structures on which the SSI domain has been found. There are 13 instances of this domain found in the PDB. Note that there may be multiple copies of the domain in a single PDB structure, since many structures contain multiple copies of the same protein seqence.

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