Protein Kinase Inhibitor Peptide (PKI)

The protein kinase inhibitors (PKIs) are a family of proteins which act as pseudosubstrates for PKA by binding to the PKA catalytic subunits, blocking its kinase activity, modulating the extent, and duration of PKA-mediated signaling events. PKI family includes three genes encoding small, heat-stable inhibitors, PKIα, PKIβ, and PKIγ. Each PKI isoform contains a PKA inhibitory domain and a nuclear export domain, enabling PKI to both inhibit PKA and remove it from the nucleus. PKIα is highly expressed in skeletal muscle, heart, cerebral cortex, and cerebellum, whereas PKIβ is most highly expressed in testis, with a small amount of expression in brain and little to none elsewhere. PKIγ mRNA is widely expressed and found most highly expressed in heart and testis. Some tissues possess multiple isoforms of PKI, in which case the expression pattern is cell specific. For example, PKIα is localized to the Sertoli cells and PKIβ is localized to the germ cells in the testis.