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.Supported by grants from the Generalitat Valenciana (PrometeoII/2014/029) and Ministerio de Economía y Competitividad (BFU2014-58229-P to VR; BFU2012-33364 and BFU2015-66360-P to AC; FPI contract to LT) of Spain, and to EC FP7/2007-2013 BioStruct-X (grant agreement no. 283570, proposal 7687) for synchrotron access