A fundamental problem in proteomics is the identification of protein complexes and their components. We have used analytical ultracentrifugation with a fluorescence detection system (AU-FDS) to precisely and rapidly identify translation complexes in the yeast Saccharomyces cerevisiae. Following a one-step affinity purification of either poly(A)-binding protein (PAB1) or the large ribosomal subunit protein RPL25A in conjunction with GFP-tagged yeast proteins/RNAs, we have detected a 77S translation complex that contains the 80S ribosome, mRNA, and components of the closed-loop structure, eIF4E, eIF4G, and PAB1. This 77S structure, not readily observed previously, is consistent with the monosomal translation complex. The 77S complex abundance decreased with translational defects and following the stress of glucose deprivation that causes translational stoppage. By quantitating the abundance of the 77S complex in response to different stress conditions that block translation initiation, we observed that the stress of glucose deprivation affected translation initiation primarily by operating through a pathway involving the mRNA cap binding protein eIF4E whereas amino acid deprivation, as previously known, acted through the 43S complex. High salt conditions (1M KCl) and robust heat shock acted at other steps. The presumed sites of translational blockage caused by these stresses coincided with the types of stress granules, if any, which are subsequently formed.

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Protein synthesis is a highly efficient process and is under exacting control. Yet, the actual abundance of translation factors present in translating complexes and how these abundances change during the transit of a ribosome across an mRNA remains unknown. Using analytical ultracentrifugation with fluorescent detection we have determined the stoichiometry of the closed-loop translation factors for translating ribosomes. A variety of pools of translating polysomes and monosomes were identified, each containing different abundances of the closed-loop factors eIF4E, eIF4G, and PAB1 and that of the translational repressor, SBP1. We establish that closed-loop factors eIF4E/eIF4G dissociated both as ribosomes transited polyadenylated mRNA from initiation to elongation and as translation changed from the polysomal to monosomal state prior to cessation of translation. eIF4G was found to particularly dissociate from polyadenylated mRNA as polysomes moved to the monosomal state, suggesting an active role for translational repressors in this process. Consistent with this suggestion, translating complexes generally did not simultaneously contain eIF4E/eIF4G and SBP1, implying mutual exclusivity in such complexes. For substantially deadenylated mRNA, however, a second type of closed-loop structure was identified that contained just eIF4E and eIF4G. More than one eIF4G molecule per polysome appeared to be present in these complexes, supporting the importance of eIF4G interactions with the mRNA independent of PAB1. These latter closed-loop structures, which were particularly stable in polysomes, may be playing specific roles in both normal and disease states for specific mRNA that are deadenylated and/or lacking PAB1. These analyses establish a dynamic snapshot of molecular abundance changes during ribosomal transit across an mRNA in what are likely to be critical targets of regulation.

We have previously identified 55 nonribosomal proteins in PAB1-mRNP complexes in Saccharomyces cerevisiae using mass spectrometric analysis. Because one of the inherent limitations of mass spectrometry is that it does not inform as to the size or type of complexes in which the proteins are present, we consequently used analytical ultracentrifugation with fluorescent detection system (AU-FDS) to determine which proteins are present in the 77S monosomal translation complex that contains minimally the closed-loop structure components (eIF4E, eIF4G, and PAB1), mRNA, and the 40S and 60S ribosomes. We assayed by AU-FDS analysis 33 additional PAB1-mRNP factors but found that only five of these proteins were present in the 77S translation complex: eRF1, SLF1, SSD1, PUB1, and SBP1. eRF1 is involved in translation termination, SBP1 is a translational repressor, and SLF1, SSD1, and PUB1 are known mRNA binding proteins. Many of the known P body/stress granule proteins that associate with the PAB1-mRNP were not present in the 77S translation complex, implying that P body/stress granules result from significant protein additions after translational cessation. These data inform that AU-FDS can clarify protein complex identification that remains undetermined after typical immunoprecipitation and mass spectrometric analyses.

Acral and mucosal melanomas, the two most common subtypes of melanoma in China, exhibit different genetic alterations and biologic behavior compared with other subtypes of melanomas. The purpose of this study was to identify the genetic alterations in patients with acral or mucosal melanomas in southern China. Fluorescence in situ hybridization (FISH), immunohistochemistry (IHC) analysis, polymerase chain reaction (PCR), and quantitative real-time reverse transcriptase PCR (qRT-PCR) were used to assess the anaplastic lymphoma kinase (ALK) break points. Furthermore, a mass spectrometry-based genotyping platform was used to analyze 30 acral melanomas and 28 mucosal melanomas to profile 238 known somatic mutations in 19 oncogenes. ALK break points were identified in four acral cases (6.9%). Eight (13.8%) cases harbored BRAF mutations, six (10.3%) had NRAS mutations, four (6.9%) had KIT mutations, two (3.5%) had EGFR mutations, two (3.5%) had KRAS mutations, two (3.5%) had MET mutations, one (1.7%) had an HRAS mutation, and one (1.7%) had a PIK3CA mutation. Two cases exhibited co-occurring mutations, and one case with a BRAF mutation had a translocation in ALK. This study represents a comprehensive and concurrent analysis of the major recurrent oncogenic mutations involved in melanoma cases from southern China. These data have implications for both clinical trial designs and therapeutic strategies.