Glycogenic activity of R6, a protein phosphatase 1 regulatory subunit, is modulated by the laforin-malin complex

Protein phosphatase type 1 (PP1) plays a major role in the regulation of glycogen biosynthesis. PP1 is recruited to sites of glycogen formation by its binding to specific targeting subunits. There, it dephosphorylates different enzymes involved in glycogen homeostasis leading to an activation of glycogen biosynthesis. Regulation of these targeting subunits is crucial, as excess of them leads to an enhancement of the action of PP1, which results in glycogen accumulation. In this work we present evidence that PPP1R3D (R6), one of the PP1 glycogenic targeting subunits, interacts physically with laforin, a glucan phosphatase involved in Lafora disease, a fatal type of progressive myoclonus epilepsy. Binding of R6 to laforin allows the ubiquitination of R6 by the E3-ubiquitin ligase malin, what targets R6 for autophagic degradation. As a result of the action of the laforin-malin complex on R6, its glycogenic activity is downregulated. Since R6 is expressed in brain, our results suggest that the laforin-malin complex downregulates the glycogenic activity of R6 present in neuron cells to prevent glycogen accumulation.