Positional proteomics reveals differences in N-terminal proteoform stability.

To understand the impact of alternative translation initiation on a proteome, we performed a proteome-wide study on protein turnover using positional proteomics and ribosome profiling to distinguish between N-terminal proteoforms of individual genes. By combining pulsed SILAC with N-terminal COFRADIC, we monitored the stability of 1,941 human N-terminal proteoforms, including 147 N-terminal proteoform pairs that originate from alternative translation initiation, alternative splicing or incomplete processing of the initiator methionine. N-terminally truncated proteoforms were less abundant than canonical proteoforms and often displayed altered stabilities, likely attributed to individual protein characteristics, including intrinsic disorder, but independent of N-terminal amino acid identity or truncation length. We discovered that the removal of initiator methionine by methionine aminopeptidases reduced the stability of processed proteoforms, while susceptibility for N-terminal acetylation did not seem to influence protein turnover rates. Taken together, our findings reveal differences in protein stability between N-terminal proteoforms and point to a role for alternative translation initiation and co-translational initiator methionine removal, next to alternative splicing, in the overall regulation of proteome homeostasis.

PROTEOFORMER: deep proteome coverage through ribosome profiling and MS integration.

An increasing amount of studies integrate mRNA sequencing data into MS-based proteomics to complement the translation product search space. However, several factors, including extensive regulation of mRNA translation and the need for three- or six-frame-translation, impede the use of mRNA-seq data for the construction of a protein sequence search database. With that in mind, we developed the PROTEOFORMER tool that automatically processes data of the recently developed ribosome profiling method (sequencing of ribosome-protected mRNA fragments), resulting in genome-wide visualization of ribosome occupancy. Our tool also includes a translation initiation site calling algorithm allowing the delineation of the open reading frames (ORFs) of all translation products. A complete protein synthesis-based sequence database can thus be compiled for mass spectrometry-based identification. This approach increases the overall protein identification rates with 3% and 11% (improved and new identifications) for human and mouse, respectively, and enables proteome-wide detection of 5'-extended proteoforms, upstream ORF translation and near-cognate translation start sites. The PROTEOFORMER tool is available as a stand-alone pipeline and has been implemented in the galaxy framework for ease of use.

The proteome under translational control.

A single eukaryotic gene can give rise to a variety of protein forms (proteoforms) as a result of genetic variation and multilevel regulation of gene expression. In addition to alternative splicing, an increasing line of evidence shows that alternative translation contributes to the overall complexity of proteomes. Identifying the repertoire of proteins and micropeptides expressed by alternative selection of (near-)cognate translation initiation sites and different reading frames however remains challenging with contemporary proteomics. MS-enabled identification of proteoforms is expected to benefit from transcriptome and translatome data by the creation of customized and sample-specific protein sequence databases. Here, we focus on contemporary integrative omics approaches that complement proteomics with DNA- and/or RNA-oriented technologies to elucidate the mechanisms of translational control. Together, these technologies enable to map the translation (initiation) landscape and more comprehensively define the inventory of proteoforms raised upon alternative translation, thus assisting in the (re-)annotation of genomes.

A proteogenomics approach integrating proteomics and ribosome profiling increases the efficiency of protein identification and enables the discovery of alternative translation start sites.

Next-generation transcriptome sequencing is increasingly integrated with MS to enhance MS-based protein and peptide identification. Recently, a breakthrough in transcriptome analysis was achieved with the development of ribosome profiling (ribo-seq). This technology is based on the deep sequencing of ribosome-protected mRNA fragments, thereby enabling the direct observation of in vivo protein synthesis at the transcript level. In order to explore the impact of a ribo-seq-derived protein sequence search space on MS/MS spectrum identification, we performed a comprehensive proteome study on a human cancer cell line, using both shotgun and N-terminal proteomics, next to ribosome profiling, which was used to delineate (alternative) translational reading frames. By including protein-level evidence of sample-specific genetic variation and alternative translation, this strategy improved the identification score of 69 proteins and identified 22 new proteins in the shotgun experiment. Furthermore, we discovered 18 new alternative translation start sites in the N-terminal proteomics data and observed a correlation between the quantitative measures of ribo-seq and shotgun proteomics with a Pearson correlation coefficient ranging from 0.483 to 0.664. Overall, this study demonstrated the benefits of ribosome profiling for MS-based protein and peptide identification and we believe this approach could develop into a common practice for next-generation proteomics.

N-terminal proteomics and ribosome profiling provide a comprehensive view of the alternative translation initiation landscape in mice and men.

Usage of presumed 5'UTR or downstream in-frame AUG codons, next to non-AUG codons as translation start codons contributes to the diversity of a proteome as protein isoforms harboring different N-terminal extensions or truncations can serve different functions. Recent ribosome profiling data revealed a highly underestimated occurrence of database nonannotated, and thus alternative translation initiation sites (aTIS), at the mRNA level. N-terminomics data in addition showed that in higher eukaryotes around 20% of all identified protein N termini point to such aTIS, to incorrect assignments of the translation start codon, translation initiation at near-cognate start codons, or to alternative splicing. We here report on more than 1700 unique alternative protein N termini identified at the proteome level in human and murine cellular proteomes. Customized databases, created using the translation initiation mapping obtained from ribosome profiling data, additionally demonstrate the use of initiator methionine decoded near-cognate start codons besides the existence of N-terminal extended protein variants at the level of the proteome. Various newly identified aTIS were confirmed by mutagenesis, and meta-analyses demonstrated that aTIS reside in strong Kozak-like motifs and are conserved among eukaryotes, hinting to a possible biological impact. Finally, TargetP analysis predicted that the usage of aTIS often results in altered subcellular localization patterns, providing a mechanism for functional diversification.

Matrix metalloproteinase (MMP)-2 and MMP-9 expression in tumor infiltrating CD3 lymphocytes from women with endometrial cancer.

OBJECTIVE: In this study, we hypothesized that not only endothelial malignant cells but also lymphocytes infiltrating tumor epithelium, in patients with endometrial cancer, could be an important source of the gelatinases (matrix metalloproteinase [MMP]-2 and MMP-9) extensive production, which in turn, may facilitate tumor cells infiltration and progression due to the extracellular matrix degradation. MATERIALS AND METHODS: First, we isolated lymphocytes from the endometrial carcinoma samples taken from 41 patients who were operated on and from healthy endometrial tissue taken of the same patients after histological verification. Then, we detected the level of CD3-positive cells in endometrial tissues by flow cytometry. Simultaneously, we studied the messenger RNA expression of MMP-2 and MMP-9 in the isolated cells from malignant and unchanged endometrial tissues. Using immunohistochemistry, we compared the protein expression of MMP-2, MMP-9, and CD3 in the studied samples. RESULTS: We showed the enhanced abundance of CD3 lymphocytes both by flow cytometry and immunohistochemistry in the samples from malignant tissues. The expression of MMP-9 in the endometrial carcinoma was increased significantly at the protein level but not at the messenger RNA level. We could not observe any differences concerning MMP-2 expression in both methods of detection. CONCLUSIONS: CD-3 lymphocytes significantly infiltrate endometrial cancer tissue, but they do not seem to be the source of enhanced metalloproteinases 2 and 9 expression in the tumor environment. Still, owing to the immunohistochemistry staining, we could show the significant increase of MMP-9 protein in the very close vicinity of tumor-infiltrating CD3 lymphocytes. Could it be the result of CD3 lymphocyte action, or is it just the imperfection of the detecting method we used? This remains unclear. Further studies explaining the role of tumor infiltrating lymphocytes in mediating the endometrial cancer milieu are needed.