Protein kinase C iota (PKCι) is a serine-threonine kinase that affects many cellular processes, including growth, proliferation, and motility.
PKCι associates with two discrete polarity complexes; one containing the polarity proteins Par-3 and Par-6 (PAR complex) and the other contains Crumbs, Stardust and PatJ (the Crbs complex). Both complexes are found in vertebrates and invertebrates where they are crucial for maintaining apical-basal polarity.
Loss of cell polarity is often associated with aggressive epithelial cell cancers, and PKCι is frequently found in abnormally high abundance in epithelial cancers. PKCι is therefore a validated oncogene and potential drug target.
This year we reported the discovery and mode of action of potent PKCi-selective chemical inhibitors (Kjaer et al., 2013; Biochem J. 451(2): 329-42).
These inhibitors were identified in collaboration with the Cancer Research Technology (CRT) Discovery Lab and the Protein Phosphorylation Group.
Prior to publication, few isoform-specific chemical biology tools were available to inhibit PKCι catalytic activity. We determined a crystal structure of PKCι bound to a representative compound, CRT0066854, revealing how it binds within the nucleotide cleft and displaces a crucial Asn-Phe-Asp motif found in many AGC kinases (Figure 1A).
CRT0066854 inhibits phosphorylation of the PKCι substrate LLGL2 in cell lines and exhibits phenotypic effects in a range of cell-based assays.
This compound has been sent out to numerous academic laboratories for use as a chemical biology tool to modulate PKCι/PKCζ activity in vitro and in vivo in a variety of cancer models. During the inspection of the PKCι-CRT0066854 inhibitor structure, we noticed an invariant motif on the surface of PKCi.
In collaboration with the Protein Phosphorylation Group, we explored a possible role for this RIPR motif in protein-protein interaction leading to the discovery that this motif is required to engage the PKCι substrate LLGL2 (Linch et al., 2013; Sci Signal. 6(293): ra82).
Surprisingly, PKCι mutants associated with human cancer were found that target this motif highlighting the importance of PKCι substrate recruitment.