YnbB is a CdsA Paralogue Dedicated to Biosynthesis of Glycolipid MPIase Involved in Membrane Protein Integration

MPIase is a glycolipid involved in protein integration in E. coli. Recently, we identified CdsA, a CDP-diacylglycerol (CDP-DAG) synthase, as a biosynthetic enzyme for MPIase. YnbB is a CdsA paralogue with a highly homologous C-terminal half. Under CdsA-depleted conditions, YnbB overproduction restored MPIase expression, but not phospholipid biosynthesis. YnbB complemented the growth defect of the cdsA knockout when Tam41p, a mitochondrial CDP-DAG synthase, was co-expressed, suggesting that YnbB possesses sufficient activity for MPIase biosynthesis, but not for phospholipid biosynthesis. Consistently, a chimera consisting of the CdsA N-terminal half and the YnbB C-terminal half (CdsA-N-YnbB-C) complemented the cdsA knockout by itself, but a chimera consisting of the YnbB N-terminal half and the CdsA C-terminal half (YnbB-N-CdsA-C) required co-expression of Tam41p for the complementation. The biosynthetic rate for CDP-DAG in CdsA and CdsA-N-YnbB-C was much faster than that in YnbB and YnbB-N-CdsA-C, indicating that the N-terminal half of CdsA accelerates CDP-DAG biosynthesis to give the fast cell growth. Therefore, the role of YnbB seems to be as a backup for MPIase biosynthesis, suggesting that YnbB is dedicated to MPIase biosynthesis. A mutant with a high pH-sensitive CdsA8 was unable to grow even under permissive conditions when the ynbB gene was deleted, supporting its auxiliary role in the CdsA function.