Alternative splicing of the bicistronic gene molybdenum cofactor synthesis 1 (MOCS1) uncovers a novel mitochondrial protein maturation mechanism.

Molybdenum cofactor (Moco) biosynthesis is a highly conserved multistep pathway. The first step, the conversion of GTP to cyclic pyranopterin monophosphate (cPMP), requires the bicistronic gene molybdenum cofactor synthesis 1 (MOCS1). Alternative splicing of MOCS1 within exons 1 and 9 produces four different N-terminal and three different C-terminal products (type I-III). Type I splicing results in bicistronic transcripts with two open reading frames, of which only the first, MOCS1A, is translated, whereas type II/III splicing produces MOCS1AB proteins. Here, we first report the cellular localization of alternatively spliced human MOCS1 proteins. Using fluorescence microscopy, fluorescence spectroscopy, and cell fractionation experiments, we found that depending on the alternative splicing of exon 1, type I splice variants (MOCS1A) either localize to the mitochondrial matrix (exon 1a) or remain cytosolic (exon 1b). MOCS1A proteins required exon 1a for mitochondrial translocation, but fluorescence microscopy of MOCS1AB variants (types II and III) revealed that they were targeted to mitochondria independently of exon 1 splicing. In the latter case, cell fractionation experiments displayed that mitochondrial matrix import was facilitated via an internal motif overriding the N-terminal targeting signal. Within mitochondria, MOCS1AB underwent proteolytic cleavage resulting in mitochondrial matrix localization of the MOCS1B domain. In conclusion, MOCS1 produces two functional proteins, MOCS1A and MOCS1B, which follow different translocation routes before mitochondrial matrix import for cPMP biosynthesis involving both proteins. MOCS1 protein maturation provides a novel alternative splicing mechanism that ensures the coordinated mitochondrial targeting of two functionally related proteins encoded by a single gene.