Studies on cyanobacterial protein PipY shed light on structure, potential functions, and vitamin B6 -dependent epilepsy.

The Synechococcus elongatus COG0325 gene pipY functionally interacts with the nitrogen regulatory gene pipX. As a first step toward a molecular understanding of such interactions, we characterized PipY. This 221-residue protein is monomeric and hosts pyridoxal phosphate (PLP), binding it with limited affinity and losing it upon incubation with D-cycloserine. PipY crystal structures with and without PLP reveal a single-domain monomer folded as the TIM barrel of type-III fold PLP enzymes, with PLP highly exposed, fitting a role for PipY in PLP homeostasis. The mobile PLP phosphate-anchoring C-terminal helix might act as a trigger for PLP exchange. Exploiting the universality of COG0325 functions, we used PipY in site-directed mutagenesis studies to shed light on disease causation by epilepsy-associated mutations in the human COG0325 gene PROSC.