Structural characterization of PTG and PTG/PP1 protein complex

PTG (protein targeting to glycogen) is a scaffolding protein that is involved in the activation of glycogen synthesis by bringing PP1 (type 1 protein phosphatase) to its substrates. It is proposed as a target for the treatment of Lafora disease (LD), a genetic disorder manifested by catastrophic teenage onset of progressive myoclonus epilepsy. In healthy neurons, PTG is downregulated by the laforin-malin complex resulting in very low glycogen production. Mutations in malin or laforin causes accumulation of PTG, which promotes glycogen synthesis by directing PP1 to glycogen synthase and glycogen phosphorylase. This results in the appearance of neurotoxic inclusion bodies formed by insolublepolyglucosans called Lafora bodies (LB), which are ultimately responsible for Lafora disease (LD). In LD mice models knocking out PTG resulted in a nearly complete disappearance of LB and resolution of neurodegeneration and myoclonic epilepsy, indicating that small molecules interfering with the PTG/PP1 interaction emerges as an excellent therapeutic strategy for LD. Up to date, there was no structural data of PTG and PTG/PP1 complex allowing for identification of potential druggable pockets. We present our efforts to obtain structural information of PTG-CBM21 and PTG/PP1 complex. We were able to set up protein production and crystallization protocols, which resulted in obtaining good quality crystals allowing for structural studies. Herein we report the first structures of human PTG: PTG-CBM21 in complex with β-cyclodextrin, PP1 in complex with PTG N-terminal peptide containing the conserved binding motif RVXF and finally PTG/PP1 holoenzyme. Our findings contribute to elucidating the interplay mechanism between PTG and PP1 and provide the basis for further structural analysis in order to identify druggable pocket.