Pentti Somerharju
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Researcher at Department of Biochemistry and Developmental Biology, Faculty of Medicine, University of Helsinki
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Cardiolipin (CL) is an essential phospholipid for mitochondrial structure and function. Here we present a small mitochondrial protein, NERCLIN, as a negative regulator of CL homeostasis and mitochondrial ultrastructure. Primate-specific NERCLIN is expressed ubiquitously from GRPEL2 locus on a tightly regulated low level, but induced by heat stress. NERCLIN overexpression severely disrupts mitochondrial cristae structure and induces mitochondrial fragmentation. Proximity labeling suggested interactions of NERCLIN with CL synthesis and prohibitin complexes on the matrix side of the inner mitochondrial membrane. Lipid analysis indicated that NERCLIN regulates mitochondrial CL content. The regulation may occur directly through interaction with PTPMT1, a proximal partner on the CL synthesis pathway, as its product phosphatidylglycerol was also reduced by NERCLIN. We propose that NERCLIN contributes to stress-induced adaptation of mitochondrial dynamics and turnover by regulating the mitochondrial CL content. Our findings add NERCLIN to the group of recently identified small mitochondrial proteins with important regulatory functions.
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Glycerophospholipid (GPL) homeostasis in eukaryotic cells is thought to be maintained via biosynthesis, degradation and acyl chain remodeling. Here we provide evidence for an additional process termed head-group remodeling where other GPLs, when in excess, are rapidly converted to phosphatidylcholine and triacylglycerol. Mass spectrometric studies showed the formation of diacylglycerol, but not phosphatidic acid, from the exogenous GPL thus indicating that the first step is catalyzed by a phospholipase C-type enzyme. Consistently, triacylglycerol formation was significantly inhibited by the knock-down of several PLCs, but not phospholipase Ds. Second, we found that each exogenous GPL strongly inhibited the synthesis of the corresponding endogenous GPL class. Based on these and previous data we hypothesize how mammalian cells could coordinate the multiple processes contributing to GPL homeostasis in mammalian cells. In conclusion, this study provides the first evidence that head group remodeling plays an important role in GPL homeostasis in mammalian cells.