Extracellular Protein Phosphorylation, the Neglected Side of the Modification

Abstract

The very existence of extracellular phosphorylation has been questioned for a long time, although casein phosphorylation was discovered a century ago. In addition, several modification sites localized on secreted proteins or on extracellular or lumenal domains of transmembrane proteins have been catalogued in large scale phosphorylation analyses, though in most such studies this aspect of cellular localization was not considered. Our review presents examples when additional analyses were performed on already public data sets that revealed a wealth of information about this "neglected side" of the modification. We also sum up accumulated knowledge about extracellular phosphorylation, including the discovery of Golgi-residing kinases and the special difficulties encountered in targeted analyses. We hope future phosphorylation studies will not ignore the existence of phosphorylation outside of the cell, and further discoveries will shed more light on its biological role. Molecular & Cellular Proteomics 16: 10.1074/mcp.O116.064188, 1-7, 2017.

Insights into an unusual Auxiliary Activity 9 family member lacking the histidine brace motif of lytic polysaccharide monooxygenases

“…In addition, in the Fungi Phosphorylation Database (FPD) (50) documented Ser phosphorylation sites in fungal species (including Aspergillus) can be found which are consistent with the LsAA9B sequence surrounding the S25 phosphorylation, supporting the notion that phosphorylation could play a role in fungal plant biomass degradation. It is known that proteins can be phosphorylated in the secretory pathway (51,52), in mammalian cells by the Fam20C kinase (53,54), and phosphorylation does appear to play a role extracellularly during some biological events (e.g. during microbial host infection (55)(56)(57)).…”

Aspergillus niger Prolyl Endoprotease for Hydrogen–Deuterium Exchange Mass Spectrometry and Protein Structural Studies

“…Another acidic Pro-specific protease, Neprosin, 49 from the Nepenthes plant genus, has been recently used for proteomics and histone mapping. 36 …”

Biochemical evidence of furin specificity and potential for phospho-regulation at Spike protein S1/S2 cleavage site in SARS-CoV2 but not in SARS-CoV1 or MERS-CoV

“…Importantly, although not sufficiently studied, it is known that besides cyto-and nucleoplasm, protein phosphorylation occurs also in the extracellular space, and in lumens of ER and Golgi (Klement and Medzihradszky, 2017;Tagliabracci et al, 2013;Yalak and Vogel, 2012). For example, one of the first discovered phosphoproteins was casein, a true secreted protein (Levene and Hill, 1933).…”

“…However, the Spike protein, except for its short C-terminal tail, is considered to be facing the endoplasmic reticulum (ER) or the Golgi lumen during the viral replication cycle (Lontok et al, 2004) and is not present in cytoplasm or the nucleus, where most of the kinase signaling is taking place. Nevertheless, despite still being a poorly studied field, it is well known that protein phosphorylation does not only occur on cytoplasmic and nuclear proteins, but also takes place on secreted proteins in ER and Golgi lumen, and also in the extracellular space (Dartt et al, 1996;Klement and Medzihradszky, 2017;Preisinger et al, 2004;Yalak and Vogel, 2012). Strikingly, supporting evidence for the idea of Spike phospho-regulation arises from a recent report that confirmed more than ten in vivo phosphorylated sites in the Spike protein of SARS-CoV2 (Davidson et al 2020).…”