Interaction of nucleosome assembly proteins abolishes nuclear localization of DGKI by attenuating its association with importins

Diacylglycerol kinase (DGK) is involved in the regulation of lipid-mediated signal transduction through the metabolism of a second messenger diacylglycerol. Of the DGK family, DGKI, which contains a nuclear localization signal, localizes mainly to the nucleus but translocates to the cytoplasm under pathological conditions. However, the detailed mechanism of translocation and its functional significance remain unclear. To elucidate these issues, we used a proteomic approach to search for protein targets that interact with DGKI. Results show that nucleosome assembly protein (NAP) 1-like 1 (NAP1L1) and NAP1-like 4 (NAP1L4) are identified as novel DGKI binding partners. NAP1Ls constitutively shuttle between the nucleus and the cytoplasm in transfected HEK293 cells. The molecular interaction of DGKI and NAP1Ls prohibits nuclear import of DGKI because binding of NAP1Ls to DGKI blocks import carrier proteins, Qip1 and NPI1, to interact with DGKI, leading to cytoplasmic tethering of DGKI. In addition, overexpression of NAP1Ls exerts a protective effect against doxorubicin-induced cytotoxicity. These findings suggest that NAP1Ls are involved in a novel molecular basis for the regulation of nucleocytoplasmic shuttling of DGKI and provide a clue to examine functional significance of its translocation under pathological conditions.