serpin family B member 9, cytoplasmic antiproteinase 3
Cytoplasmic antiproteinase 3 (CAP-3; peptidase inhibitor 9/PI-9, Spi6, or testicular tissue protein Li 180) is an intracellular inhibitor of granzyme B (grB) that protects cytotoxic lymphocytes from grB-mediated death. It is also thought to be expressed in accessory immune cells, including dendritic cells (DCs), although there is some debate about this. Overexpression of serpin B9 may prevent cytotoxic T-lymphocytes from eliminating certain tumor cells. A pseudogene of this gene is found on chromosome 6. Diseases associated with serpin B9 include chronic obstructive pulmonary disease (COPD) and oral squamous cell carcinoma (OSCC). The ovalbumin family of serpins (ov-serpins) is a family of closely related proteins, whose members can be secreted (ovalbumin), cytosolic (leukocyte elastase inhibitor, LEI), or targeted to both compartments (plasminogen activator inhibitor 2, PAI-2). It also characterized by N- and C-terminal extensions, the absence of a signal peptide, and a Ser rather than an Asn residue at the penultimate position. The ov-serpins corresponds to clade B of the serpin superfamily. In general, SERine Proteinase INhibitors (serpins) exhibit conformational polymorphism shifting from native to cleaved, latent, delta, or polymorphic forms. Many serpins, such as antitrypsin and antichymotrypsin, function as serine protease inhibitors which regulate blood coagulation cascades. Non-inhibitory serpins perform many diverse functions such as chaperoning proteins or transporting hormones. Serpins are of medical interest because mutants can cause blood clotting disorders, emphysema, cirrhosis, and dementia. A classification based on evolutionary relatedness has resulted in the assignment of serpins to 16 clades designated A-P along with some orphans.
Comment:depending on the conformational state, the RC loop is surface accessible in the active form or buried and inserted as the central beta strand in the inactive form.
Jiyao W et al.(2023). "The conserved domain database in 2023.",