Cystatin

Proteinase inhibitor I25, cystatin
Crystal structure of an immunomodulatory salivary cystatin from the soft tick Ornithodoros moubata from PDB entry 3L0R.[1]
Identifiers
Symbol	Prot_inh_cystat
Pfam	PF00031
Pfam clan	CL0121
InterPro	IPR000010
SMART	SM00043
PROSITE	PDOC00259
Available protein structures: Pfam   structures / ECOD   PDB RCSB PDB; PDBe; PDBj PDBsum structure summary PDB PDB: 1a67​ PDB: 1a90​ PDB: 1cew​ PDB: 1cyu​ PDB: 1cyv​ PDB: 1dvc​ PDB: 1dvd​ PDB: 1eqk​ PDB: 1g96​ PDB: 1gd3​

The cystatins are a family of cysteine protease inhibitors which share a sequence homology and a common tertiary structure of an alpha helix lying on top of an anti-parallel beta sheet. The family is subdivided as described below.

Cystatins show similarity to fetuins, kininogens, histidine-rich glycoproteins and cystatin-related proteins.^[2][3][4] Cystatins mainly inhibit peptidase enzymes (another term for proteases) belonging to peptidase families C1 (papain family) and C13 (legumain family). They are known to mis-fold to form amyloid deposits and are implicated in several diseases.^{[citation needed]}