Modification in hydrophobic packing of HAMP domain induces a destabilization of the auto-phosphorylation site in the histidine kinase CpxA

Distinct role of HAMP and HAMP-like linker domains in regulating the activity of Hik1p, a hybrid histidine kinase 3 from Magnaporthe oryzae

Nik1 orthologs or group III hybrid histidine kinases (HHK3) represent a unique cytoplasmic osmosensor that act upstream of HOG/p38 MAPK pathway in fungi. It is an important molecular target for developing new antifungal agents against human pathogens. HHK3 orthologs contain a linear array of alternative HAMP and HAMP-like linker domains (poly-HAMP) in the N-terminal region. HAMP domains are quite common in prokaryotic histidine kinases where it mostly functions as signal transducer mediating conformational changes in the kinase domains. In contrast, poly-HAMP in HHK3 acts as a sensor and signal transducer to regulate histidine kinase activity. However, the mechanistic detail of this is poorly understood. Interestingly, recent studies indicate that the poly-HAMP-mediated regulation of the kinase activity varies among the orthologs. Hik1 is an important HHK3 ortholog from fungus Magnaporthe oryzae. In this paper, we aimed to decipher the role HAMP and HAMP-like linker domains in regulating the activity of Hik1p. We show that Hik1p acts as a bona fide osmosensor and negatively regulates the downstream HOG/p38 MAPK pathway in Saccharomyces cerevisiae. Our data suggest a differential role of the HAMP domains in the functionality of Hik1p. Most interestingly, the deletion of individual domains in poly-HAMP resulted in distinct active forms of Hik1p and thereby indicating that the poly-HAMP domain, instead of acting as on-off switch, regulates the histidine kinase activity by transition through multiple conformational states.

Histidine kinases and the missing phosphoproteome from prokaryotes to eukaryotes

Protein phosphorylation is the most common type of post-translational modification in eukaryotes. The phosphoproteome is defined as the complete set of experimentally detectable phosphorylation sites present in a cell's proteome under various conditions. However, we are still far from identifying all the phosphorylation sites in a cell mainly due to the lack of information about phosphorylation events involving residues other than Ser, Thr and Tyr. Four types of phosphate-protein linkage exist and these generate nine different phosphoresidues-pSer, pThr, pTyr, pHis, pLys, pArg, pAsp, pGlu and pCys. Most of the effort in studying protein phosphorylation has been focused on Ser, Thr and Tyr phosphorylation. The recent development of 1- and 3-pHis monoclonal antibodies promises to increase our understanding of His phosphorylation and the kinases and phosphatases involved. Several His kinases are well defined in prokaryotes, especially those involved in two-component system (TCS) signaling. However, in higher eukaryotes, NM23, a protein originally characterized as a nucleoside diphosphate kinase, is the only characterized protein-histidine kinase. This ubiquitous and conserved His kinase autophosphorylates its active site His, and transfers this phosphate either onto a nucleoside diphosphate or onto a protein His residue. Studies of NM23 protein targets using newly developed anti-pHis antibodies will surely help illuminate the elusive His phosphorylation-based signaling pathways. This review discusses the role that the NM23/NME/NDPK phosphotransferase has, how the addition of the pHis phosphoproteome will expand the phosphoproteome and make His phosphorylation part of the global phosphorylation world. It also summarizes why our understanding of phosphorylation is still largely restricted to the acid stable phosphoproteome, and highlights the study of NM23 histidine kinase as an entrée into the world of histidine phosphorylation.