Cupid PKI


Function and Interactions

Protein kinase inhibitor (PKI) is an endogenous thermostable protein that inhibits PKA (cAMP-dependent protein kinase) activity and plays an important physiological role in regulating PKA function. PKA plays a critical role in numerous cellular processes. This is particularly evident in the nervous system where PKA is involved in neurotransmitter release, gene transcription, and synaptic plasticity. Through its interaction on PKA, PKI modulates neuronal synaptic activity, is involved in morphogenesis and symmetrical left-right axis formation and also plays a role in regulating gene expression induced by PKA. (1, 2)

The PKI protein binds directly to the catalytic subunit of PKA acting as a pseudo-substrate to cause PKA kinase inhibition. The inhibition is greatly enhanced by the presence of Mg2+ and ATP which the kinase binds, as it ‘locks in’ the inhibitor. PKIs have also been shown to cause translocation of the PKA catalytic subunit from the nucleus to the cytoplasm and themselves contain a nuclear export signal (3,4)

Three distinct PKI isoforms (PKIalpha, PKIbeta, PKIgamma) have been identified with each isoform present in the brain but showing variation in expression patterns (5,6). Each isoform possesses the capability to inhibit PKA at different strengths (7), but is not known what other functional differences might exist between these isoforms. Recently a requirement for PKIbeta and its inhibition of PKA (but not PKIalpha or PKIgamma isoforms) has been shown during neuronal differentiation of P19 cells suggesting other factors influence the system (8).

Finally, the 20 amino acid peptide region (PKI alpha 6-24) is effective at inhibiting PKA in vitro and, when linked to a cell-penetrating peptide vehicle, in vivo. Consistent with its PKA inhibitory action, this CPP-PKI blocked aggregation in wild-type Dictyostelium cells and, at appropriate concentrations, rescued the phenotype of a Dictyostelium mutant that has constitutively high PKA activity (9). We are offering this PKI alpha region linked to Cupid as Product: Cupid-PKI.


(1) PKA: lessons learned after twenty years. Taylor SS, Zhang P, Steichen JM, Keshwani MM, Kornev AP. Biochim Biophys Acta. 2013 Jul;1834(7):1271-8. doi: 10.1016/j.bbapap.2013.03.007

(2) Protein kinase inhibitor peptide (PKI): a family of endogenous neuropeptides that modulate neuronal cAMP-dependent protein kinase function. Dalton GD, Dewey WL. Neuropeptides. 2006 Feb;40(1):23-34

(3) Effect of metal ions on high-affinity binding of pseudosubstrate inhibitors to PKA.Zimmermann B, Schweinsberg S, Drewianka S, Herberg FW. Biochem J. 2008 Jul 1;413(1):93-101. doi: 10.1042/BJ20071665.

(4) Purification and structural study of the beta form of human cAMP-dependent protein kinase inhibitor. Jin R, Dai L, Zheng J, Ji C. Eur J Biochem. 2004 May;271(9):1768-73.

(5) Differential expression of mRNAs for protein kinase inhibitor isoforms in mouse brain. Seasholtz AF, Gamm DM, Ballestero RP, Scarpetta MA, Uhler MD. Proc Natl Acad Sci U S A. 1995 Feb 28;92(5):1734-8.

(6) The expression and intracellular distribution of the heat-stable protein kinase inhibitor is cell cycle regulated. Wen W, Taylor SS, Meinkoth JL. J Biol Chem. 1995 Feb 3;270(5):2041-6.

(7) Isoform-specific differences in the potencies of murine protein kinase inhibitors are due to nonconserved amino-terminal residues. Gamm DM, Uhler MD. J Biol Chem. 1995 Mar 31;270(13):7227-32.

(8) Ascl1-induced neuronal differentiation of P19 cells requires expression of a specific inhibitor protein of cyclic AMP-dependent protein kinase. Huang HS, Turner DL, Thompson RC, Uhler MD. J Neurochem. 2012 Mar;120(5):667-83. doi: 10.1111/j.1471-4159.2011.07332.x. Epub 2011 Jun 24.

(9) Use of a penetratin-linked peptide in Dictyostelium. Ryves WJ, Harwood AJ. Mol Biotechnol. 2006 Jun;33(2):123-32.

Cupid MDM2 Cargoes

Cupid PKI Cargo

Human cAMP-dependent protein kinase inhibitor alpha$=protalign&blast_rank=4&RID=9TJ304S1014

76 amino acids

PKI Alpha Sequence


Cupid Cargo PKI


Amino acids 6 to 25 (20 amino acids)

PKA catalytic unit pseudo-substrate binding motif