Biological Function
Physiologically SPP processes signal peptides of classical MHC class I preproteins. A nine amino acid-long cleavage fragment is then presented on HLA-E receptors and modulates the activity of natural killer cells.
SPP also plays a pathophysiological role; it cleaves the structural nucleocapsid protein (also known as core protein) of the Hepatitis C virus and thus influences viral reproduction rate.
In mice, a nonamer peptide originating from the SPP protein serves as minor histocompatibility antigen HM13 that plays a role in transplant rejection
The homologous proteases SPPL2A and SPPL2B promote the intramembrane cleavage of TNFα in activated dendritic cells and might play an immunomodulatory role. For SPPL2c and SPPL3 no substrates are known.
SPPs do not require cofactors as demonstrated by expression in bacteria and purification of a proteolytically active form. The C-terminal region defines the functional domain, which is in itself sufficient for proteolytic activity.
Read more about this topic: Signal Peptide Peptidase
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