Detection and minimization of H-phosphonate side reaction during phosphopeptide synthesis by a post-assembly global phosphorylation strategy

Dissecting the role of protein phosphorylation: a chemical biology toolbox

Protein phosphorylation is a crucial regulator of protein and cellular function, yet, despite identifying an enormous number of phosphorylation sites, the role of most is still unclear. Each phosphoform, the particular combination of phosphorylations, of a protein has distinct and diverse biological consequences. Aberrant phosphorylation is implicated in the development of many diseases. To investigate their function, access to defined protein phosphoforms is essential. Materials obtained from cells often are complex mixtures. Recombinant methods can provide access to defined phosphoforms if site-specifically acting kinases are known, but the methods fail to provide homogenous material when several amino acid side chains compete for phosphorylation. Chemical and chemoenzymatic synthesis has provided an invaluable toolbox to enable access to previously unreachable phosphoforms of proteins. In this review, we selected important tools that enable access to homogeneously phosphorylated protein and discuss examples that demonstrate how they can be applied. Firstly, we discuss the synthesis of phosphopeptides and proteins through chemical and enzymatic means and their advantages and limitations. Secondly, we showcase illustrative examples that applied these tools to answer biological questions pertaining to proteins involved in signal transduction, control of transcription, neurodegenerative diseases and aggregation, apoptosis and autophagy, and transmembrane proteins. We discuss the opportunities and challenges in the field.

The synthesis of phosphopeptides

Phosphorylation and dephosphorylation are key events in protein expression and regulation and in signal transduction. Phosphopeptides are very useful reagents for the study of these processes, and have been used to great advantage in the study of phosphatase substrate specificity, SH2 domain ligand specificity, and protein-protein interactions. Furthermore, the advent of cell-permeable peptide carriers, such as those from the antennapedia homeodomain and the HIV TAT transcription factor, has allowed the study of intracellular events, thus underscoring the utility of these reagents. In this paper we review methods for the synthesis of phosphopeptides with the emphasis on the preparation of phosphoamino acid building blocks.