PLP Homeostasis Protein Is Required for Efficient Maturation of Aromatic Amino Acid Aminotransferase in Salmonella enterica.

Pyridoxal 5'-phosphate (PLP) is an essential cofactor required for metabolic functions including amino acid biosynthesis. While the pathways of PLP synthesis are well characterized, it is unclear how newly synthesized PLP is delivered to apo PLP-dependent enzymes (PLP-DEs) in the cell. The highly conserved PLP Homeostasis Protein (PLPHP) is hypothesized to act as a PLP carrier that traffics the cofactor to PLP-DEs. Prior studies have shown transfer of PLP between PLPHP homologs and PLP-DEs in vitro. Data herein support a role for this transfer in vivo. This study utilizes a Salmonella enterica mutant strain with a synthetic requirement for PLPHP homolog, YggS. Genetic and biochemical analyses suggest that the synthetic phenotype of a yggS aspC mutant results from inefficient maturation of the aromatic amino acid aminotransferase, TyrB. This requirement for YggS can be bypassed with accumulation of B6 vitamer PMP, or upregulation of pyridoxal kinase (PdxK). Using TyrB as a target, we provide the first demonstration that YggS of S. enterica can mediate transfer of PLP to a PLP-DE in vitro. In total, this study defines multiple in vivo maturation pathways for TyrB and provides a working model for the role of YggS in the maturation of TyrB.