Abstract
Sacsin is a 520-kDa protein mutated in the early-onset neurodevelopmental and neurodegenerative disease autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS). The C terminus of the protein contains an HEPN (higher eukaryotes and prokaryotes nucleotide-binding) domain of unknown function. Here, we determined the high-resolution 1.9-Å crystal structure of the HEPN domain from human sacsin. The structure is composed of five parallel α-helices with a large loop of several short helical segments. Two HEPN protomers assemble as a dimer to form a large positively charged cavity at the dimer interface that binds GTP and other nucleotides. The crystal structure reveals that the ARSACS N4549D mutation disrupts dimerization and protein folding. This study provides novel insights into the oligomerization state of sacsin and functions that are lost in mutations that cause ARSACS.
Highlights
JUNE 10, 2011 VOLUME 286 NUMBER 23 which is highly expressed in neurons
Sacsin has been identified as a potential substrate of the ubiquitin ligase Ube3A responsible for Angelman syndrome, a neurodevelopmental disorder with a motor component that is similar to that seen in autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) [5]
The high-resolution crystal structure reveals the molecular basis of the defect in the sacsin missense mutation N4549D, which causes ARSACS [11]
Summary
Dimeric assembly with a large positively charged cavity that is unique to the HEPN domain of sacsin. Isothermal titration calorimetry and NMR titrations showed that the HEPN domain binds GTP with low-micromolar affinity. Loss of these functions by either missense or truncation mutations leads to the neurological disease ARSACS [2, 11, 12]
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