Diacylglycerol kinase δ associates with receptor for activated C kinase 1, RACK1

The δ-isozyme (type II) of diacylglycerol kinase (DGK) is known to positively regulate growth factor receptor signaling. DGKδ, which is distributed to clathrin-coated vesicles, interacts with DGKδ itself, protein kinase C and AP2α. To search for additional DGKδ-interacting proteins, we screened a yeast two-hybrid cDNA library from HepG2 cells using aa 896–1097 of DGKδ as a bait. We identified aa 184–317 (WD40 repeats 5–7) of receptor for activated C kinase 1 (RACK1), which interacts with various important signaling molecules, as a novel binding partner of DGKδ. Co-immunoprecipitation analysis, using COS-7 cells co-expressing RACK1 and DGKδ, revealed that RACK1 selectively interacted with DGKδ, but not with type I DGKs, in mammalian cells. The interaction was dynamically regulated by phorbol ester. Intriguingly, DGKδ appeared to recruit RACK1 to clathrin-coated vesicles and co-localized with RACK1. These results suggest that DGKδ serves as an adaptor protein to regulate the localization of the versatile scaffold protein, RACK1.