Structured and disordered regions of Ataxin-2 contribute differently to the specificity and efficiency of mRNP granule formation

Abstract

Ataxin-2 (ATXN2) is a gene implicated in spinocerebellar ataxia type II (SCA2), amyotrophic lateral sclerosis (ALS) and Parkinsonism. The encoded protein is a therapeutic target for ALS and related conditions. ATXN2 (or Atx2 in insects) can function in translational activation, translational repression, mRNA stability and in the assembly of mRNP-granules, a process mediated by intrinsically disordered regions (IDRs). Previous work has shown that the LSm (Like-Sm) domain of Atx2, which can help stimulate mRNA translation, antagonizes mRNP-granule assembly. Here we advance these findings through a series of experiments on Drosophila and human Ataxin-2 proteins. Results of Targets of RNA Binding Proteins Identified by Editing (TRIBE), co-localization and immunoprecipitation experiments indicate that a polyA-binding protein (PABP) interacting, PAM2 motif of Ataxin-2 may be a major determinant of the mRNA and protein content of Ataxin-2 mRNP granules. Transgenic Experiments with transgenic Drosophila indicate that while the Atx2-LSm domain may protect against neurodegeneration, structured PAM2- and unstructured IDR- interactions both support Atx2-induced cytotoxicity. Taken together, the data lead to a proposal for how Ataxin-2 interactions are remodelled during translational control and how structured and non-structured interactions contribute differently to the specificity and efficiency of RNP granule condensation as well as to neurodegeneration. Mutations in genes that promote protein aggregation are primarily associated with neurodegenerative diseases. RNA-binding proteins, crucial for gene regulation and cellular stress response, are also major constituents of protein aggregate-rich mRNP granules. Recent studies aim to understand the molecular mechanisms governing the formation of stress granules identified Intrinsically Disordered Regions (IDRs) within mRNP granule proteins as a key factor that facilitates the formation of amyloid-like structures. A deeper understanding of stress granule dynamics holds promise for elucidating neurodegenerative disease mechanisms. Ataxin-2, is an RNA-binding protein implicated in neurodegenerative disorders like spinocerebellar ataxia type II, amyotrophic lateral sclerosis, and Parkinsonism. Reducing the levels of Ataxin-2 levels or deleting its IDR prevents RNP aggregation and offers neuroprotection in animal models of neurodegenerative diseases. We show that deleting the structured LSm domain of Ataxin-2, which impedes granule assembly, enhances cytotoxicity. Moreover, specific molecular interactions involving Ataxin-2 PAM2 motif binding to polyA-binding protein is crucial for recruiting mRNAs and other proteins into RNP granules. These interactions not only shape granule composition but also influence disease progression. Our findings propose a model illustrating the dynamic remodeling of Ataxin-2 interactions, emphasizing the distinct roles of structured and unstructured domains in governing mRNP-granule condensation and neurodegeneration.