PhD Student, University of Iowa
I study how polyglutamine tracts in transcriptional regulators may modulate protein function.
Polyglutamine (poly-Q) tracts are protein features prone to vary in length (number of repeats of the amino acid glutamine) due to the instability of the underlying repetitive DNA sequences. In some proteins, such as the human huntingtin protein, expansion of the poly-Q tracts beyond ~40 glutamines causes neurodegenerative disease (1). Despite the potential risk of disease causing expansions, poly-Q tracts are surprisingly prevalent in eukaryotic proteomes and are specifically enriched in transcriptional regulators (2), suggesting that poly-Q tracts are involved in the critically important cellular process of regulating gene expression. Accordingly, poly-Q tracts have been implicated in regulatory protein-protein interactions. The addition or removal of poly-Q tracts in proteins has been shown to directly promote (3) or disrupt (4) interactions respectively. Poly-Q tract length has also been shown to correlate with the extent of transcriptional activation. A direct correlation between tract length and transcriptional activation has been observed by adding glutamine tracts of increasing length to a synthetic transcription factor (TF) (5). Based on the propensity of poly-Q tracts to mediate protein-protein interactions, tract length dependent gene activation could be explained by longer poly-Q tracts forming more stable regulatory interactions resulting in increased transcriptional activation. To investigate the contributions of poly-Q tracts to regulatory protein-protein interactions I am studying yeast (S. cerevisiae) Gal11 (Med15) and its interactors, which include mediator subunits, transcription factors (TFs) and chromatin remodeling complex proteins. Gal11 is a poly-Q rich subunit of the RNA Polymerase II mediator complex with three polyglutamine tracts that vary in length among strains of S. cerevisiae. The interactions between Gal11 and DNA-bound regulatory proteins promote proper positioning of Mediator and enable recruitment of RNA polymerase II to stress response and other target genes. I hypothesize that there is a function for poly-Q tracts with the capacity to vary in length in transcriptional regulators as a dynamic modulator of protein-protein interaction strength and therefore gene expression.  Gatchel and Zoghbi (2005) Nature Rev Genet, 6,  Schaefer et al. (2012) Nucleic Acids Res, 40,  Stott et al. (1995) PNAS, 92,  Nucifora et al. (2001) Science, 291,  Atanesyan et al. (2012) Biol Chem, 393
Abstract: Expanded runs of consecutive trinucleotide CAG repeats encoding polyglutamine (polyQ) stretches are observed in the genes of a large number of patients with different genetic diseases such as Huntington's and several Ataxias. Protein aggregation, which is a key feature of most of these diseases, is thought to be triggered by these expanded polyQ sequences in disease-related proteins. However, polyQ tracts are a normal feature of many human proteins, suggesting that they have an important cellular function. To clarify the potential function of polyQ repeats in biological systems, we systematically analyzed available information stored in sequence and protein interaction databases. By integrating genomic, phylogenetic, protein interaction network and functional information, we obtained evidence that polyQ tracts in proteins stabilize protein interactions. This happens most likely through structural changes whereby the polyQ sequence extends a neighboring coiled-coil region to facilitate its interaction with a coiled-coil region in another protein. Alteration of this important biological function due to polyQ expansion results in gain of abnormal interactions, leading to pathological effects like protein aggregation. Our analyses suggest that research on polyQ proteins should shift focus from expanded polyQ proteins into the characterization of the influence of the wild-type polyQ on protein interactions.
Pub.: 31 Jan '12, Pinned: 29 Jun '17
Abstract: Microsatellite repeats are genetically unstable and subject to expansion and shrinkage. A subset of them, triplet repeats, can occur within the coding region and specify homomeric tracts of amino acids. Polyglutamine (polyQ) tracts are enriched in eukaryotic regulatory proteins, notably transcription factors, and we had shown before that they can contribute to transcriptional activation in mammalian cells. Here we generalize this finding by also including evolutionarily divergent organisms, namely, Drosophila and baker's yeast. In all three systems, Gal4-based model transcription factors were more active if they harbored a polyQ tract, and the activity depended on the length of the tract. By contrast, a polyserine tract was inactive. PolyQs acted from either an internal or a C-terminal position, thus ruling out a merely structural 'linker' effect. Finally, a two-hybrid assay in mammalian cells showed that polyQ tracts can interact with each other, supporting the concept that a polyQ-containing transcription factor can recruit other factors with polyQ tracts or glutamine-rich activation domains. The widespread occurrence of polyQ repeats in regulatory proteins suggests a beneficial role; in addition to the contribution to transcriptional activity, their genetic instability might help a species to adapt to changing environmental conditions in a potentially reversible manner.
Pub.: 26 May '12, Pinned: 29 Jun '17
Abstract: Excessive expansions of glutamine (Q)-rich repeats in various human proteins are known to result in severe neurodegenerative disorders such as Huntington's disease and several ataxias. However, the physiological role of these repeats and the consequences of more moderate repeat variation remain unknown. Here, we demonstrate that Q-rich domains are highly enriched in eukaryotic transcription factors where they act as functional modulators. Incremental changes in the number of repeats in the yeast transcriptional regulator Ssn6 (Cyc8) result in systematic, repeat-length-dependent variation in expression of target genes that result in direct phenotypic changes. The function of Ssn6 increases with its repeat number until a certain threshold where further expansion leads to aggregation. Quantitative proteomic analysis reveals that the Ssn6 repeats affect its solubility and interactions with Tup1 and other regulators. Thus, Q-rich repeats are dynamic functional domains that modulate a regulator's innate function, with the inherent risk of pathogenic repeat expansions.
Pub.: 11 Aug '15, Pinned: 29 Jun '17
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