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ArgRS, MetRS, and SRRM2 dependent changes in mRNA splicing and exon usage

ABSTRACT: Arginine is involved in inflammation and amino acid signaling, in part through the mTORC1 pathway. In cell-based assays and in the mouse, we found that arginine regulates nuclear levels of arginyl-tRNA synthetase (ArgRS) and that nuclear ArgRS interacts and co-localizes with the Serine/Arginine Repetitive Matrix Protein 2 (SRRM2), a spliceosomal protein crucial for the formation of nuclear speckle condensates. Arginine depletion or ArgRS knock down, both of which decreased nuclear ArgRS levels, mobilized SRRM2 for trafficking from nuclear condensates and altered mRNA processing and splice junction usage while minimally affecting expression of other cellular RNAs. These splice junction changes included a subset that were inversely correlated with SRRM2 knock down induced changes and the affected genes encompassed components of the mTORC1 pathway. This inverse correlation suggests that the nuclear ArgRS-SRRM2 interaction regulates SRRM2 nuclear trafficking and alternative splicing in the physiological response to changed arginine levels resulting from inflammation.

ORGANISM(S): Homo sapiens

PROVIDER: GSE165513 | GEO | 2023/04/03

REPOSITORIES: GEO

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Project description:The nucleus of higher eukaryotes is a highly compartmentalized and dynamic organelle consisting of several biological condensates that regulate gene expression at multiple levels. First reported more than 100 years ago by Ramón y Cajal, nuclear speckles (NS) are among the most prominent of such condensates, which were independently rediscovered multiple times and are also known as interchromatin granule clusters (ICGs), splicing speckles, or SC35 domains. Despite their prevalence, research on the function of NS is virtually restricted to colocalization analyses, since an organizing core, without which NS cannot form, remains unidentiﬁed. The monoclonal antibody SC35, which was raised against a spliceosomal extract, is a frequently used reagent to mark NS since its debut in 1990. Despite its proliﬁc use, and the consensus regarding its primary target as SRSF2, clear inconsistencies between observations made with mAb SC35, and transgenic SRSF2 constructs in the same experiment are noted. Intrigued by these disparities, we carried out a systematic re-characterization of this monoclonal antibody and its cellular targets. Unexpectedly, we found no evidence for SC35 mAb recognizing SRSF2 in human cells. In contrast, our results show that the 35 kDa namesake protein recognized by SC35 mAb is SRSF7, a related but distinct SR protein. More importantly, using mass-spectrometry, CRISPR-mediated knock-ins, immunoblotting and ﬂuorescence microscopy, we show that the main target of SC35 mAb is SRRM2, a large (300 kDa), spliceosome-associated protein with prominent intrinsically disordered regions (IDRs) that sharply localizes to NS. Analysis of protein length evolution among metazoa revealed a peculiar IDR extension speciﬁcally for SRRM2 and SON in vertebrates, a hallmark of condensate formation. Combining these results, we tested a long-standing question in the study of NS: whether NS are formed around a speciﬁc core, or various SR-proteins self-assemble into a phase-separated compartment without the need for a deﬁned core. Here we show that, the elusive core of NS is formed by SON and SRRM2, since depletion of SON leads only to a partial disassembly of NS, while combined depletion of SON together with SRRM2, but not other NS associated factors, or depletion of SON in a cell line where IDRs of SRRM2 are genetically deleted, leads to a near-complete dissolution of NS. This work, therefore, paves the way to study the role of NS under diverse physiological and stress conditions.

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ArgRS, MetRS, and SRRM2 dependent changes in mRNA splicing and exon usage

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