Malin decreases glycogen accumulation by promoting the degradation of protein targeting to glycogen (PTG).
Lafora disease (LD) is an autosomal recessive neurodegenerative disease that results in progressive myoclonus epilepsy and death. LD is caused by mutations in either the E3 ubiquitin ligase malin or the dual specificity phosphatase laforin. A hallmark of LD is the accumulation of insoluble glycogen in the cytoplasm of cells from most tissues. Glycogen metabolism is regulated by phosphorylation of key metabolic enzymes. One regulator of this phosphorylation is protein targeting to glycogen (PTG/R5), a scaffold protein that binds both glycogen and many of the enzymes involved in glycogen synthesis, including protein phosphatase 1 (PP1), glycogen synthase, phosphorylase, and laforin. Overexpression of PTG markedly increases glycogen accumulation, and decreased PTG expression decreases glycogen stores. To investigate if malin and laforin play a role in glycogen metabolism, we overexpressed PTG, malin, and laforin in tissue culture cells. We found that expression of malin or laforin decreased PTG-stimulated glycogen accumulation by 25%, and co-expression of malin and laforin abolished PTG-stimulated glycogen accumulation. Consistent with this result, we found that malin ubiquitinates PTG in a laforin-dependent manner, both in vivo and in vitro, and targets PTG for proteasome-dependent degradation. These results suggest an additional mechanism, involving laforin and malin, in regulating glycogen metabolism.
Worby
Carolyn A
CA
Department of Pharmacology, University of California at San Diego, La Jolla, California 92093-0721, USA.
Gentry
Matthew S
MS
Dixon
Jack E
JE
D000818:Animals; D016466:CHO Cells; D002352:Carrier Proteins / Q000378:metabolism* / Q000502:physiology*; D006224:Cricetinae; D003412:Cricetulus; D006003:Glycogen / Q000378:metabolism*; D006801:Humans; D006868:Hydrolysis; D047468:Immunoprecipitation; D047908:Intracellular Signaling Peptides and Proteins; D010749:Phosphoprotein Phosphatases / Q000378:metabolism*; D054558:Protein Tyrosine Phosphatases, Non-Receptor / Q000378:metabolism; D025801:Ubiquitin / Q000378:metabolism; D044767:Ubiquitin-Protein Ligases
D002352:Carrier Proteins; D047908:Intracellular Signaling Peptides and Proteins; C491294:PPP1R3C protein, human; D025801:Ubiquitin; D006003:Glycogen; C478960:NHLRC1 protein, human; D044767:Ubiquitin-Protein Ligases; D010749:Phosphoprotein Phosphatases; D054558:Protein Tyrosine Phosphatases, Non-Receptor; C114990:EPM2A protein, human