A journey from phosphotyrosine to phosphohistidine and beyond

Abstract

Protein phosphorylation is a reversible post-translational modification. Nine of the 20 natural amino acids in proteins can be phosphorylated, but most of what we know about the roles of protein phosphorylation has come from studies of serine, threonine, and tyrosine phosphorylation. Much less is understood about the phosphorylation of histidine, lysine, arginine, cysteine, aspartate, and glutamate, so-called non-canonical phosphorylations. Phosphohistidine (pHis) was discovered 60 years ago as a mitochondrial enzyme intermediate; since then, evidence for the existence of histidine kinases and phosphohistidine phosphatases has emerged, together with examples where protein function is regulated by reversible histidine phosphorylation. pHis is chemically unstable and has thus been challenging to study. However, the recent development of tools for studying pHis has accelerated our understanding of the multifaceted functions of histidine phosphorylation, revealing a large number of proteins that are phosphorylated on histidine and implicating pHis in a wide range of cellular processes.

Keywords: histidine kinase; phosphohistidine; phosphohistidine phosphatase.

Figure 1:. Enzymatic phosphorylation and dephosphorylation of histidine residues in proteins.

Histidine protein kinases transfer the γ-phosphate from ATP onto the 1- or 3-nitrogren of the imidazole ring of histidine residues in proteins. The NME1/2 histidine kinases transfer the phosphate in a two-step process via a phosphoenzyme intermediate resulting in inversion of phosphate chirality (this step is not depicted in the figure). Phosphohistidine phosphatases (PHPs) hydrolyze the phosphate from the 1- or 3-position of the histidine imidazole ring releasing a free phosphate ion (P_i). In the case of the PGAM5 PHP, the P-N bond in the target pHis is attacked by the active site His of PGAM5, forming a transient pHis intermediate that is then hydrolyzed.

Figure 2:. Timeline of key discoveries in the field of protein histidine phosphorylation.

Beginning with the report of covalent phosphorylation of proteins in 1906, and the identification of phosphoserine in proteins in 1932, the timeline shows some important milestones in histidine phosphorylation following the discovery of phosphohistidine (pHis) as a protein modification in 1962. Abbreviations: ACLY: ATP citrate lyase; KCa3.1: calcium-activated potassium channel 3.1; NDPK-A: nucleoside diphosphate kinase A; PHPT1: phosphohistidine phosphatase 1; P’ase: phosphatase; P’ation: phosphorylation; SCS: succinyl CoA synthase; TCS: two-component system.