Cystatin

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. 
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.
Types.
The cystatin family includes:
Plant cystatins.
Plant cystatins have special characteristics which permit them to be classified in a special class called Phytocystatin. One is the presence of a N-terminal alpha-helix, present only in plant cystatins. Phytocystatins are involved in several process, including plant germination and defense. van Wyk et al. found some 19 different cystatins similar to oryzacystatin-I in the soybean along with related cysteine proteases.
Membrane permeability.
Chicken cystatin quickly passed the membrane of MCF-10A neo T cells and inhibited cathepsin B when it was acylated with fatty acyl residues of 6-18 carbon atoms.