For 15 years the mission of PhosphoSitePlus® (PSP, https://www.phosphosite.org) has been to provide comprehensive information and tools for the study of mammalian post-translational modifications (PTMs). The number of unique PTMs in PSP is now more than 450 000 from over 22 000 articles and thousands of MS datasets. The most important areas of growth in PSP are in disease and isoform informatics. Germline mutations associated with inherited diseases and somatic cancer mutations have been added to the database and can now be viewed along with PTMs and associated quantitative information on novel ‘lollipop' plots. These plots enable researchers to interactively visualize the overlap between disease variants and PTMs, and to identify mutations that may alter phenotypes by rewiring signaling networks. We are expanding the sequence space to include over 30 000 human and mouse isoforms to enable researchers to explore the important but understudied biology of isoforms. This represents a necessary expansion of sequence space to accommodate the growing precision and depth of coverage enabled by ongoing advances in mass spectrometry. Isoforms are aligned using a new algorithm. Exploring the worlds of PTMs and disease mutations in the entire isoform space will hopefully lead to new biomarkers, therapeutic targets, and insights into isoform biology.
The p53 protein is a major mediator of the cellular response to genotoxic stress and is a crucial suppressor of tumor formation. In a variety of organisms, p53 and its paralogs, p63 and p73, each encode multiple protein isoforms through alternative splicing, promoters, and translation start sites. The function of these isoforms in development and disease are still being defined. Here, we evaluate the apoptotic potential of multiple isoforms of the single p53 gene in the genetic model Drosophila melanogaster. Most previous studies have focused on the p53A isoform, but it has been recently shown that a larger p53B isoform can induce apoptosis when overexpressed. It has remained unclear, however, whether one or both isoforms are required for the apoptotic response to genotoxic stress. We show that p53B is a much more potent inducer of apoptosis than p53A when overexpressed. Overexpression of two newly identified short isoforms perturbed development and inhibited the apoptotic response to ionizing radiation. Analysis of physiological protein expression indicated that p53A is the most abundant isoform, and that both p53A and p53B can form a complex and co-localize to sub-nuclear compartments. In contrast to the overexpression results, new isoformspecific loss-of-function mutants indicated that it is the shorter p53A isoform, not full-length p53B, that is the primary mediator of pro-apoptotic gene transcription and apoptosis after ionizing radiation. Together, our data show that it is the shorter p53A isoform that mediates the apoptotic response to DNA damage, and further suggest that p53B and shorter isoforms have specialized functions.
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