Jeremy A. M. Morgan

Protein phosphorylation is a central signalling mechanism in eukaryotic cells. The scope of this post-translational modification includes protein pyro- and polyphosphorylation. Here we report the discovery of another mode of phosphorylation: protein oligophosphorylation. Using site-specifically phosphorylated and pyrophosphorylated nucleoside diphosphate kinase A (NME1), the effects of these modifications on enzyme activity were investigated. Phosphorylation, and more so pyrophosphorylation, on Thr94 reduced the nucleoside diphosphate kinase activity. Nevertheless, both phosphoprotein and pyrophosphoprotein catalysed their own oligophosphorylation—up to the formation of a hexaphosphate chain—using ATP as a cofactor. Oligophosphorylation was critically dependent on the catalytic histidine residue His118, and cryogenic electron microscopy analysis of the modified proteins suggests an intramolecular phosphoryl transfer mechanism. Oligophosphorylation of NME1 in biochemical samples, and in cell lysates, was further confirmed using mass spectrometry, and was found to promote a new set of protein interactions. Our results highlight the complex nature of phosphoregulation, and the methods described here provide the opportunity to investigate the impact of this unusual modification in the future.

Protein phosphorylation is a central regulatory mechanism in eukaryotic cell signaling, and was recently expanded to include protein pyrophosphorylation and protein polyphosphorylation. Here, we report the discovery of yet another mode of phosphorylation – protein oligophosphorylation. Using site-specifically phosphorylated and pyrophosphorylated nucleoside diphosphate kinase A (NME1), the effects of these modifications on enzyme activity were investigated. Phosphorylation, and more so pyrophosphorylation, on threonine 94 notably reduced the nucleoside diphosphate kinase activity. Nevertheless, both phosphoprotein and pyrophosphoprotein were able to catalyze their own oligophosphorylation – up to the formation of a hexaphosphate chain – using ATP as a co-factor. This reaction was critically dependent on the catalytic histidine residue H118, and cryo-EM analysis of the differently modified proteins suggests an intramolecular phosphoryl transfer, likely via a phosphohistidine intermediate. Oligophosphorylation of NME1 in biochemical samples, as well as cell lysates, was further confirmed using mass spectrometry, and oligophophorylation promoted a new set of protein interactions. Our results highlight the complex nature of phosphoregulation, and the methods described here provide the opportunity to investigate the impact of this novel modification in the future.

Reversible protein phosphorylation is a central signaling mechanism in eukaryotes. Although mass-spectrometry-based phosphoproteomics has become routine, identification of non-canonical phosphorylation has remained a challenge. Here we report a tailored workflow to detect and reliably assign protein pyrophosphorylation in two human cell lines, providing, to our knowledge, the first direct evidence of endogenous protein pyrophosphorylation. We manually validated 148 pyrophosphosites across 71 human proteins, the most heavily pyrophosphorylated of which were the nucleolar proteins NOLC1 and TCOF1. Detection was consistent with previous biochemical evidence relating the installation of the modification to inositol pyrophosphates (PP-InsPs). When the biosynthesis of PP-InsPs was perturbed, proteins expressed in this background exhibited no signs of pyrophosphorylation. Disruption of PP-InsP biosynthesis also significantly reduced rDNA transcription, potentially by lowering pyrophosphorylation on regulatory proteins NOLC1, TCOF1 and UBF1. Overall, protein pyrophosphorylation emerges as an archetype of non-canonical phosphorylation and should be considered in future phosphoproteomic analyses.

Reversible protein phosphorylation is a central signaling mechanism in eukaryotic cells. While the identification of canonical phosphorylation sites using mass-spectrometry (MS) based proteomics has become routine, annotation of non-canonical phosphorylation has remained a challenge. Here, we report a tailored pyrophosphoproteomics workflow to detect and reliably assign protein pyrophosphorylation in two human cell lines, providing the first direct evidence of endogenous protein pyrophosphorylation. Detection of protein pyrophosphorylation was reproducible, specific and consistent with previous biochemical evidence relating the installation of the modification to inositol pyrophosphates (PP-InsPs). We manually validated 148 pyrophosphosites across 71 human proteins, the most heavily pyrophosphorylated of which were the nucleolar proteins NOLC1 and TCOF1. A predictive workflow based on the MS data set was established to recognize putative pyrophosphorylation sequences, and UBF1, a nucleolar protein incompatible with the proteomics method, was biochemically shown to undergo pyrophosphorylation. When the biosynthesis of PP-InsPs was perturbed in a model cell line, proteins expressed in this background exhibited lower levels of pyrophosphorylation. Disruption of PP-InsP biosynthesis also significantly reduced rDNA transcription, potentially by lowering pyrophosphorylation on regulatory proteins NOLC1, TCOF1, and UBF1. Overall, protein pyrophosphorylation emerges as an archetype of non-canonical phosphorylation, and should be considered in future phosphoproteomic analyses.

An important step in elucidating the function of protein post-translational modifications (PTMs) is gaining access to site-specifically modified, homogeneous samples for biochemical characterization. Protein pyrophosphorylation is a poorly characterized PTM, and here a chemical approach to obtain pyrophosphoproteins is reported. Photo-labile phosphorimidazolide reagents were developed for selective pyrophosphorylation, affinity-capture, and release of pyrophosphoproteins. Kinetic analysis of the reaction revealed rate constants between 9.2 × 10⁻³ to 0.58 M⁻¹ s⁻¹, as well as a striking proclivity of the phosphorimidazolides to preferentially react with phosphate monoesters over other nucleophilic side chains. Besides enabling the characterization of pyrophosphorylation on protein function, this work highlights the utility of phosphoryl groups as handles for selective protein modification for a variety of applications, such as phosphoprotein bioconjugation and enrichment.