The mammalian multi-tRNA synthetase complex

Project Summary/Abstract Mammalian cells contain a cytoplasmic multi-tRNA synthetase complex (MSC) consisting of 8 aminoacyl-tRNA synthetases (AARSs) and 3 non-synthetase proteins. AARSs in the MSC function as “gene decoders” during mRNA translation, but also exhibit non-canonical functions outside the MSC. However, the assembly, structure, and function of the MSC are poorly understood. Importantly, mutations in genes encoding 7/11 constituents cause central nervous system (CNS) disorders – five cause hypomyelinating leukodystrophy (HLD), and two others cause progressive microcephaly. We will utilize state-of-the-art molecular approaches to improve our understanding of the MSC, and its potential role in neuropathology. Our proposed Multiple-PI program takes advantage of the expertise of two highly collaborative PI's – Paul Fox (Contact PI), a molecular biologist with long-term interest in tRNA synthetases and the MSC, and Valentin Gogonea (Multiple PI), a physical chemist with expertise in analysis and molecular modeling of multi-protein complexes. We will determine the quaternary structure of the MSC by cross-linking mass spectrometry (XL-MS), a state-of-the-art method that facilitates analysis of otherwise intractable complexes. To date we have found 19 inter-protein cross-links between all 11 MSC constituents, and 118 intra-protein cross-links. We have generated an initial model of the MSC that will be refined here by XL-MS experiments with expanded amino acid specificity, and by SiMPull (single-molecule pulldown) coupled with single-molecule fluorescence to determine stoichiometry. In addition, we will investigate the mechanism of assembly of the MSC. Constitutive, multi-protein complexes are thought to be assembled by domain-specific interactions between fully-formed, mature constituents (“post-translational assembly”). However, assembly of some complexes utilizes a “co-translational assembly” mechanism in which a mature constituent interacts with the nascent peptide of a partner constituent as it emerges from the ribosome. In preliminary data we show at least 10 pairs of MSC constituents interact co-translationally. We will apply these mechanistic approaches to elucidate the role of two MSC constituents in CNS diseases – genetic defects in QARS1 and EPRS1 that cause microcephaly and HLD, respectively. Our preliminary studies indicate that constituent mutation or suppression can lead to extra-MSC accumulation. Our preliminary studies have led us to propose the following hypothesis: The mammalian MSC is a compact structure assembled in part by an orderly sequence of co-translational interactions, however, mis-assembly or mutation can induce extra-MSC accumulation of constituents, with potentially deleterious downstream consequences. We will test this hypothesis by (1) determining MSC quaternary structure and component stoichiometry, and (2) determining the role of co-translational interactions in MSC formation and integrity. We anticipate that elucidation of the structure and assembly of the MSC will provide insights into mechanisms by which molecular defects in MSC constituents can cause severe pathological disturbances, in particular, debilitating disorders of the CNS.