How many human proteoforms are there?

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Abstract

<p class="first" id="P14">Despite decades of accumulated knowledge about proteins and their post-translational modifications (PTMs), numerous questions remain regarding their molecular composition and biological function. One of the most fundamental queries is the extent to which the combinations of DNA-, RNA- and PTM-level variations explode the complexity of the human proteome. Here, we outline what we know from current databases and measurement strategies including mass spectrometry–based proteomics. In doing so, we examine prevailing notions about the number of modifications displayed on human proteins and how they combine to generate the protein diversity underlying health and disease. We frame central issues regarding determination of protein-level variation and PTMs, including some paradoxes present in the field today. We use this framework to assess existing data and to ask the question, “How many distinct primary structures of proteins (proteoforms) are created from the 20,300 human genes?” We also explore prospects for improving measurements to better regularize protein-level biology and efficiently associate PTMs to function and phenotype. </p>

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Evolvability

M Kirschner, J C GERHART (1998)

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Widespread Expansion of Protein Interaction Capabilities by Alternative Splicing.

Xinping Yang, Jasmin Coulombe-Huntington, Shuli Kang … (2016)

While alternative splicing is known to diversify the functional characteristics of some genes, the extent to which protein isoforms globally contribute to functional complexity on a proteomic scale remains unknown. To address this systematically, we cloned full-length open reading frames of alternatively spliced transcripts for a large number of human genes and used protein-protein interaction profiling to functionally compare hundreds of protein isoform pairs. The majority of isoform pairs share less than 50% of their interactions. In the global context of interactome network maps, alternative isoforms tend to behave like distinct proteins rather than minor variants of each other. Interaction partners specific to alternative isoforms tend to be expressed in a highly tissue-specific manner and belong to distinct functional modules. Our strategy, applicable to other functional characteristics, reveals a widespread expansion of protein interaction capabilities through alternative splicing and suggests that many alternative "isoforms" are functionally divergent (i.e., "functional alloforms").

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Precise determination of the diversity of a combinatorial antibody library gives insight into the human immunoglobulin repertoire.

Jacob Glanville, Wenwu Zhai, Jan Berka … (2009)

Antibody repertoire diversity, potentially as high as 10(11) unique molecules in a single individual, confounds characterization by conventional sequence analyses. In this study, we present a general method for assessing human antibody sequence diversity displayed on phage using massively parallel pyrosequencing, a novel application of Kabat column-labeled profile Hidden Markov Models, and translated complementarity determining region (CDR) capture-recapture analysis. Pyrosequencing of domain amplicon and RCA PCR products generated 1.5 x 10(6) reads, including more than 1.9 x 10(5) high quality, full-length sequences of antibody variable fragment (Fv) variable domains. Novel methods for germline and CDR classification and fine characterization of sequence diversity in the 6 CDRs are presented. Diverse germline contributions to the repertoire with random heavy and light chain pairing are observed. All germline families were found to be represented in 1.7 x 10(4) sequences obtained from repeated panning of the library. While the most variable CDR (CDR-H3) presents significant length and sequence variability, we find a substantial contribution to total diversity from somatically mutated germline encoded CDRs 1 and 2. Using a capture-recapture method, the total diversity of the antibody library obtained from a human donor Immunoglobulin M (IgM) pool was determined to be at least 3.5 x 10(10). The results provide insights into the role of IgM diversification, display library construction, and productive germline usages in antibody libraries and the humoral repertoire.