Alternative splicing of bicistronic MOCS1 defines a novel mitochondrial protein maturation mechanism

Molybdenum cofactor biosynthesis is a conserved multistep pathway. The first step, the conversion of GTP to cyclic pyranopterin monophosphate (cPMP), requires bicsistronic MOCS1. Alternative splicing of MOCS1 in 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 two-domain MOCS1AB proteins. Here, we report and characterize the mitochondrial translocation of alternatively spliced MOCS1 proteins. While MOCS1A requires exon 1a for mitochondrial translocation, MOCS1AB variants target to mitochondria via an internal motif overriding the N-terminal targeting signal. Within mitochondria, MOCS1AB undergoes proteolytic cleavage resulting in mitochondrial matrix localization of the MOCS1B domain. In conclusion we found that MOCS1 produces two functional proteins, MOCS1A and MOCS1B, which follow different translocation routes before mitochondrial matrix import, where both proteins collectively catalyze cPMP biosynthesis. MOCS1 protein maturation provides a novel mechanism of alternative splicing ensuring the coordinated targeting of two functionally related mitochondrial proteins encoded by a single gene.