Mitochondrial NM23-H4/NDPk-D is Multifunctional: Fueling Mitochondrial GTPase OPA1 and Triggering Mitophagy

Abstract

NM23-H4/NDPK-D forms symmetrical homohexameric complexes in the mitochondrial inter-membrane space. The well-established function of NM23-H4 is phosphotransfer activity as a nucleoside diphosphate kinase, using mitochondrial ATP to regenerate NTPs, especially GTP. NM23-H4 also strongly binds in vitro to anionic phospholipids, mainly cardiolipin (CL), and in vivo to the mitochondrial inner membrane (MIM). Membrane-binding seems to be important for close co-localization of NM23-H4 with mitochondrial OPA1, a dynamin-like GTPase, involved in fusion of MIM. NM23-H4/OPA1 association increases GTP-loading on OPA1. Like OPA1 loss-of-function, silencing of NM23-H4, but not cytosolic NM23-H1/H2, results in mitochondrial fragmentation, reflecting fusion defects. Thus, NM23-H4 interacts with and provides GTP to OPA1, similar to what is observed for cytosolic NM23 isoforms which interact with endocytic dynamin-1 and −2 and provide GTP for efficient dynamin-mediated endocytosis (Boissan et al.2014, Science 344:1510). Such close association allows these motor proteins to work with high thermodynamic efficiency. Earlier, we have shown that NM23-H4, when fully bound simultaneously to MIM and outer membrane (MOM), loses its kinase activity, but becomes competent to support intermembrane lipid transfer. This depends on the presence of the mitochondria-specific CL, and allows CL to move from its site of synthesis, MIM, to the opposed MOM (Schlattner et al.2013, JBC 288:111). Once CL is externalized at the mitochondrial surface, it can serve as a recognition signal for the autophageal machinery, leading to the elimination of damaged mitochondria. In cells treated with a protonophoric uncoupler, CCCP, CL externalization and mitophagy are stimulated only by transfection with NM23-H4 wild-type, but not R90D-mutant, incapable of CL binding. Similarly, in mouse lung epithelial cells, knocking-down NM23-H4 suppresses CL externalization and mitophagy. These findings suggest that NM23-H4 has dual functions in bioenergetics and lipid signaling leading to autophagy. Support: FRM,ARC,GEFLUC, NIH(U19AIO6802/HL114453), HFSP(RGP0013/2014).