The von Hippel–Lindau tumour-suppressor protein interaction with protein kinase Cδ
The VHL (von Hippel–Lindau) tumour-suppressor protein forms a multi-protein complex [VCB (pVHL–elongin C–elongin B)–Cul-2 (Cullin-2)] with elongin C, elongin B, Cul-2 and Rbx1, acting as a ubiquitin-ligase (E3) and directing proteasome-dependent degradation of targeted proteins. The α-subunit of Hif1α (hypoxia-inducible factor 1α) is the principal substrate for the VCB–Cul-2 complex; however, other substrates such as aPKC (atypical protein kinase C) have been reported. In the present study, we show with FRET (fluorescence resonance energy transfer) analysis measured by FLIM (fluorescence lifetime imaging microscopy) that PKCδ and pVHL (VHL protein) interact directly in cells. This occurs through the catalytic domain of PKCδ (residues 432–508), which appears to interact with two regions of pVHL, residues 113–122 and 130–154. Despite this robust interaction, analysis of the PMA-induced proteasome-dependent degradation of PKCδ in different RCC (renal cell carcinoma) lines (RCC4, UMRC2 and 786 O) shows that there is no correlation between the degradation of PKCδ and the presence of active pVHL. Thus, in contrast with aPKC, PKCδ is not a conventional substrate of the ubiquitin-ligase complex, VCB–Cul-2, and the observed interaction between these two proteins must underlie a distinct signalling output.