Structural Mechanism of Laforin Function in Glycogen Dephosphorylation and Lafora Disease

Abstract

Glycogen is the major mammalian glucose storage cache and is critical for energy homeostasis. Glycogen synthesis in neurons must be tightly controlled, due to neuronal sensitivity to perturbations in glycogen metabolism. Lafora disease (LD) is a fatal, congenital, neurodegenerative epilepsy. Mutations in the gene encoding the glycogen phosphatase laforin result in hyperphosphorylated glycogen that forms water‐insoluble inclusions called Lafora bodies (LBs). LBs induce neuronal apoptosis and are the causative agent of LD. The mechanism of glycogen dephosphorylation by laforin and dysfunction in LD is unknown. We report the crystal structure of laforin bound to phosphoglucan product revealing its unique integrated tertiary and quaternary structure. Structure‐guided mutagenesis combined with biophysical and biochemical analyses reveal the basis for normal function of laforin in glycogen metabolism. Analyses of LD patient mutations define the mechanism by which subsets of mutations disrupt laforin function. These data provide fundamental insights connecting glycogen metabolism to neurodegenerative disease.

Regulatory Mechanisms and Novel Therapeutic Targeting Strategies for Protein Tyrosine Phosphatases

“…199−202 The three-dimensional structure, along with complementary biophysical data, suggest that laforin can exist as an antiparallel dimer, which is mediated by residues in its phosphatase domain. 203 Importantly, it was shown that the Lafora diseaseassociated mutation F321S can interrupt dimerization of laforin and reduces its phosphatase activity toward glycogen. Myotubularin related protein 2 (MTMR2), a phosphoinositide lipid phosphatase, can also form as a homodimer (Figure 10C), which is important for its function inside the cell.…”

The Toxoplasma glucan phosphatase TgLaforin utilizes a distinct functional mechanism that can be exploited by therapeutic inhibitors

“…S2A) and utilized the recombinant protein in hydrogen-deuterium exchange (HDX) mass spectrometry (MS) experiments. This technique has previously been used to illuminate conformational dynamics of proteins by demonstrating areas of high and low solvent accessibility (44,48). We first determined deuterium uptake across the entire protein from 15 to 15,000 s (Fig.…”

“…Conversely, β-sheets within the core CBM were among the structural elements with the lowest deuterium uptake. Within the DSP domain, core structural elements had low uptake, while elements of the DSP active site, known to undergo conformational changes required for substrate interaction (44,49), displayed higher uptake. These active site regions included the recognition domain, variable loop, D-loop, PTPloop, and R-motif (Figs.…”

“…Like Sex Four-2, LSF2, another plant glucan phosphatase, lacks a traditional CBM and instead binds carbohydrates through surface binding sites within its DSP domain (43). Human laforin is an antiparallel dimer with a spatially separated CBM and DSP that are each capable of binding to carbohydrates independently (44). While they are not orthologs, SEX4 and laforin are functional equivalents as complementation of laforin into Δsex4 Arabidopsis thaliana plants rescues the starch excess phenotype (24).…”

Molecular Architecture of the Inositol Phosphatase Siw14

“…Though the minimal catalytic core of a cysteinebased phosphatase consists of ∼150 residues, 59 several DSPs assemble into functional dimers (e.g., VH1, 53,60 DUSP27, 61 and laforin 55 ) and/or bear regulatory domains in the vicinity of the DSP core that are important for catalytic activity. For instance, laforin has an N-terminal carbohydrate-binding module (CBM) that is essential for catalytic activity; 55,62 DUSP1, DUSP2, DUSP6, and DUSP5 contain an N-terminal cell division cycle 25 (CDC25) homology domain; 1 PTEN has a C-terminal C2 domain; 63 and HCE1 and DUSP12 present a guanylyltransferase domain and a Zn-binding domain, respectively. 7 In many cases, extensions longer than 80 residues lacking obviously recognizable domain organization surround the DSP core and are poorly characterized in terms of both structure and regulatory function.…”