Phosphohistidine phosphatase 1 (PHPT1) is a small protein, consisting of 125 amino acids, that catalyzes the dephosphorylation of histidine but does not have any activity towards other phosphorylated amino acids. PHPT1 was identified in 2002, and is so far the only mammalian histidine phosphatase known, but still little is known about its physiological role. No mammalian histidine kinases have hitherto been identified. Phosphorylation is one of the most important ways in which the structure and activity of a protein may be changed after translation. Proteins are phosphorylated on the side chain of amino acid residues. When a hydroxyl is phosphorylated the result is a phosphoester and when a nitrogen is phosphorylated the result is a phosphoamidate. Histidine may be phosphorylated on either of the two nitrogens of the imidazole ring of the side chain. The resulting phosphoamidate bond is labile and rich in energy, which makes histidine phosphorylation highly reversible and flexible. However, histidine phosphorylation is less studied than that of the phosphoesters due to the acid lability of the phosphoamidate bond. The work described in this thesis was focused on further elucidating the physiological role of PHPT1. Amino acid residues of importance for the activity of PHPT1 were identified, and mutants with decreased phosphatase activity were produced. These mutants have been used in studies on the function of PHPT1. By using immunohistochemical methodology the localization of PHPT1 in both mouse and human tissues was determined, with mainly similar results. A general finding was that expression of PHPT1 was high in epithelial cells with short turnover time, indicating that PHPT1 may have an important role in proliferating cells. We have also developed a comparatively fast and simple screening method for determination of PHPT1 activity. Since research in this field has been hampered by the lack of efficient and practical methodology, hopefully this new method will be an asset in search of inhibitors for PHPT1, which in turn may be used for detection of the elusive mammalian histidine kinases, the finding of which may give major breakthroughs in the field.

<p>Although it has been estimated that protein histidine phosphorylation account for about 6 % of the protein phosphorylation in eukaryotic cells; the knowledge of histidine phosphorylation and dephosphorylation is still limited. Lately, studies have appeared of a mammalian 14-kDa phospho- histidine phosphatase, also named protein histidine phosphatase and molecular cloning have provided some information of its physiological role. The object of the present study was to detect the protein expression of protein histidine phosphatase, PHPT1, in mouse tissue, by using immunohistochemistry. Tissue samples from a 4-week-old mouse (heart, liver, kidney, lung, muscle, and spleen), 5-month-old mouse (testis and intestinal), 8-month-old mouse (uterus) and an embryo from 14.5 days old mouse were obtained and processed for light microscopic examination. An absorption test was also made to confirm the specificity of the antibody. The results reveal that PHPT1 is mainly expressed in epithelium, heart- and skeletal muscle. These results provide new evidences for the understanding of the function of eukaryotic histidine phosphorylation and dephosphorylation.</p><p>KEYWORDS</p><p>Phosphohistidine, dephosphorylation, protein histidine phosphatase, phosphohistidine phosphatase, protein phosphorylation</p>

Although it has been estimated that protein histidine phosphorylation account for about 6 % of the protein phosphorylation in eukaryotic cells; the knowledge of histidine phosphorylation and dephosphorylation is still limited. Lately, studies have appeared of a mammalian 14-kDa phospho- histidine phosphatase, also named protein histidine phosphatase and molecular cloning have provided some information of its physiological role. The object of the present study was to detect the protein expression of protein histidine phosphatase, PHPT1, in mouse tissue, by using immunohistochemistry. Tissue samples from a 4-week-old mouse (heart, liver, kidney, lung, muscle, and spleen), 5-month-old mouse (testis and intestinal), 8-month-old mouse (uterus) and an embryo from 14.5 days old mouse were obtained and processed for light microscopic examination. An absorption test was also made to confirm the specificity of the antibody. The results reveal that PHPT1 is mainly expressed in epithelium, heart- and skeletal muscle. These results provide new evidences for the understanding of the function of eukaryotic histidine phosphorylation and dephosphorylation. KEYWORDS Phosphohistidine, dephosphorylation, protein histidine phosphatase, phosphohistidine phosphatase, protein phosphorylation

Chronic liver diseases, which includes alcoholic liver disease (ALD), are consistently among the top 15 leading causes of death in the United States. ALD is characterized by progression from a normal liver to fatty liver disease (hepatic steatosis), which can lead to cirrhosis, alcoholic hepatitis, and liver failure. We have identified a novel role of phosphohistidine signaling, mediated through phosphohistidine phosphatase 1 (PHPT1), in the onset of hepatic steatosis. We have identified PHPT1 as a target of selective oxidation following acute ethanol exposure as well as being downregulated following chronic ethanol exposure. We mapped the oxidative modification site and developed a mass-spectrometry based phosphohistidine phosphatase assay to determine the impact of PHPT1 oxidative modification during acute ethanol exposure. To further understand the role of PHPT1 and phosphohistidine signaling during chronic ethanol exposure, we have developed PHPT1 overexpression and knockout mouse models. These mouse models were characterized using mass spectrometry-based proteomics. They were then utilized in a 10-day chronic ethanol plus binge model to determine the impact of PHPT1 expression on the onset of ethanol-induced hepatic steatosis. In addition, advanced mass spectrometry-based phenotypic characterization was performed on the treated liver tissues to determine the key regulators and canonical pathways influencing phosphohistidine signaling during chronic ethanol exposure. We have evidence to suggest that PHPT1 overexpression plays a protective role in the onset of hepatic steatosis, the PHPT1 heterozygous model is more susceptible to liver damage, and the complete knockout model is embryonically lethal. Additionally, we have identified novel pathways and regulators involved in phosphohistidine signaling during the development of ethanol-induced hepatic steatosis.