Phytepsin, a plant homolog of mammalian lysosomal pepsins.
Phytepsin, a plant homolog of mammalian lysosomal pepsins, resides in grains, roots, stems, leaves and flowers. Phytepsin may participate in metabolic turnover and in protein processing events. In addition, it highly expressed in several plant tissues undergoing apoptosis. Phytepsin contains an internal region consisting of about 100 residues not present in animal or microbial pepsins. This region is thus called a plant specific insert. The insert is highly similar to saponins, which are lysosomal sphingolipid-activating proteins in mammalian cells. The saponin-like domain may have a role in the vacuolar targeting of phytepsin. Phytepsin, as its animal counterparts, possesses a topology typical of all aspartic proteases. They are bilobal enzymes, each lobe contributing a catalytic Asp residue, with an extended active site cleft localized between the two lobes of the molecule. One lobe has probably evolved from the other through a gene duplication event in the distant past. This family of aspartate proteases is classified by MEROPS as the peptidase family A1 (pepsin A, clan AA).