Ataxin-7 Can Export from the Nucleus via a Conserved Exportin-dependent Signal

Abstract

Spinocerebellar ataxia type 7 is a progressive neurodegenerative disorder caused by a CAG DNA triplet repeat expansion leading to an expanded polyglutamine tract in the ataxin-7 protein. Ataxin-7 appears to be a transcription factor and a component of the STAGA transcription coactivator complex. Here, using live cell imaging and inverted fluorescence recovery after photobleaching, we demonstrate that ataxin-7 has the ability to export from the nucleus via the CRM-1/exportin pathway and that ataxin-7 contains a classic leucine-type nuclear export signal (NES). We have precisely defined the location of this NES in ataxin-7 and found it to be fully conserved in all vertebrate species. Polyglutamine expansion was seen to reduce the nuclear export rate of mutant ataxin-7 relative to wildtype ataxin-7. Subtle point mutation of the NES in polyglutamine expanded ataxin-7 increased toxicity in primary cerebellar neurons in a polyglutamine length-dependent manner in the context of fulllength ataxin-7. Our results add ataxin-7 to a growing list of polyglutamine disease proteins that are capable of nuclear shuttling, and we define an activity of ataxin-7 in the STAGA complex of trafficking between the nucleus and cytoplasm. Spinocerebellar ataxia type 7 (SCA7)4 is a dominantly inherited neurodegenerative disorder characterized by loss of neurons in the cerebellum, brain stem, and retina (1). SCA7 is a member of a family of neurodegenerative diseases in which a CAG DNA triplet repeat expansion results in polyglutamine expansion in the gene product (2). Other members of this polyglutamine expansion disease family include Huntington disease, spinobulbar muscle atrophy, dentatorubral pallidoluysian atrophy, and spinocerebellar ataxia types 1, 2, 3, 6, and 17 (3, 4). One unique feature of SCA7 is the loss of photoreceptor neurons in the retina leading to cone-rod dystrophy (5). The mutant ataxin-7 protein can have polyglutamine repeats from 38 to 300 residues in length (6). Ataxin-7 subcellular localization has been seen to be primarily nuclear with nuclear import signals (NLSs) defined in both the central (7), and carboxyl-terminal regions of the protein (8). Within the nucleus, ataxin-7 is known to be a subunit of the mammalian GCN5 histone acetyltransferase STAGA transcription coactivator complex (9). Ataxin-7 directly binds GCN5, and mutant ataxin-7 can inhibit the histone acetyltransferase activity of STAGA (10). Although the precise biological function of ataxin-7 is unknown, mutant ataxin-7 is known to interfere with Crx-dependent transcription of retinal photoreceptor-specific genes (11, 12). Ataxin-7 interacts with TFTC/STAGA protein subunits through a central evolutionarily conserved block of residues that have defined an ataxin-7 homology family in species ranging from human to yeast (9). In Saccharomyces cerevisiae, the yeast ataxin-7 homolog, Sgf73, is a member of the SAGA and SLIK histone acetyltransferase complexes (13). Ataxin-7 and the Crx homeodomain transcription factor interact via glutamine regions in each p...

Proteolytic Cleavage of Ataxin-7 by Caspase-7 Modulates Cellular Toxicity and Transcriptional Dysregulation

“…The ataxin-7 caspase cleavage sites that we identified are clustered at amino acids 266 and 344, an area that contains a NES (26). Ataxin-7 is found both in the cytoplasm and in the nucleus (3).…”

“…Ataxin-7 is an 892-amino acid protein with a glutamine repeat starting at amino acid position 30. Ataxin-7 contains a leucine-type NES in its N terminus (position 340 -349) (26), NLSs in its C terminus (positions 705 and 835) (25), and a functional phosphoprotein site (52). A highly conserved "zinc binding domain" between 311 and 406 of ataxin-7 may mediate its interaction with the SPT3-TAF9-ADA-GCNS acetyltransferase (STAGA) complex (29).…”

“…1) (26). The protein localizes both to the nucleus and cytoplasm and is functionally and physically associated with the SAGA complex (Spt/Ada/Gcn5 acetylase) (27), which acts as a transcriptional coactivator/adapter with histone acetyltransferase activity.…”

Molecular pathogenesis and cellular pathology of spinocerebellar ataxia type 7 neurodegeneration

“…Caspase-7 mediated cleavage would generate a short fragment containing the amino-terminus with the polyQ tract but without the nuclear export sequence (NES), potentially resulting in accumulation of a mutant ataxin-7 fragment in the nucleus. Compared to full-length mutant ataxin-7 protein, expression of a mutant ataxin-7 amino-terminal fragment similar in size to the truncation product found in vivo, had enhanced nuclear localization and cellular toxicity (44). It is not currently understood how the polyQexpanded form of ataxin-7 causes degeneration in specific neuronal populations.…”

Disrupting SUMOylation enhances transcriptional function and ameliorates polyglutamine androgen receptor–mediated disease

“…Consistent with these studies, phosphomimetic S776D ataxin-1 recapitulates toxicity, and pathogenic S776 phosphorylation and subsequent nuclear transport are prevented by association with 14-3-3 (50, 51). In SCA7, point mutations in the NES signal of polyQ ataxin-7 reduce nuclear export and exacerbate toxicity (52). Phosphomimetic mutations at Ser13 and Ser16 in polyQ huntingtin rescue disease and aggregation, while phospho-null mutations iterate pathogenesis (53).…”