Upsala Journal of Medical Sciences. 2009; 114: 65 72 ORIGINAL ARTICLE Immunohistochemical localization of phosphohistidine phosphatase PHPT1 in mouse and human tissues XIAU-QUN ZHANG1, ULLA BECKMAN SUNDH1,2, LEIF JANSSON3, O¨RJAN ZETTERQVIST1 & PIA EK1 1Department of Medical Biochemistry and Microbiology, Uppsala University, Uppsala, Sweden, 2Toxicology Division, National Food Administration, Uppsala, Sweden, and 3Department of Medical Cellbiology, Uppsala University, Uppsala, Sweden Abstract Protein histidine phosphorylation accounts for about 6% of the total protein phosphorylation in eukaryotic cells; still details concerning histidine phosphorylation and dephosphorylation are limited. A mammalian 14-kDa phosphohistidine phosphatase, also denominated PHPT1, was found 6 years ago that provided a new tool in the study of phosphohistidine phosphorylation. The localization of PHPT1 mRNA by Northern blot analysis revealed high expression in heart and skeletal muscle. The main object of the present study was to determine the PHPT1 expression on protein level in mouse tissues in order to get further information on the physiological role of the enzyme. Tissue samples from adult mice and 14.5-day-old mouse embryos were processed for immunostaining using a PHPT1-specific polyclonal antibody. The same antibody was also provided to the Swedish human protein atlas project (HPR) (http://www.proteinatlas.org/index.php). The results from both studies were essentially consistent with the previously reported expression of mRNA of a few human tissues. In addition, several other tissues, including testis, displayed a high protein expression. A salient result of the present investigation was the ubiquitous expression of the PHPT1 protein and its high expression in continuously dividing epithelial cells. Key words: Phosphohistidine phosphatase, PHPT1, PHP, phosphohistidine, dephosphorylation, HPR-project Introduction The ability of ATP to donate a phosphate group to amino acid residues in specific proteins under the influence of protein kinases has long been known. The importance of these posttranslational modifications of proteins for regulation of their function has been demonstrated, and phosphorylation is today consid- ered to be one of the most important regulatory mechanism in living organisms. One-third of the proteins found in a typical mammalian cell are phosphorylated at a given time (1). Many human diseases are associated with abnormal phosphoryla- tion of cellular proteins (2). Phosphorylation is regulated by a large number of protein kinases and phosphoprotein phosphatases. The most investigated of these enzymes phosphorylate/dephosphorylate specific serine, threonine, and/or tyrosine residues of a given protein, thereby affecting its function. Although phosphohistidine represents a signifi- cant fraction (6%) of the protein-bound phosphate, eukaryotic histidine phosphorylation has been little studied compared with serine/threonine and tyrosine phosphorylation (3 5). One reason for this is that phosphohistidine is labile under the often very acidic conditions during which protein characterization is performed. In 1991 an important discovery in histidine phosphorylation was made by the purification of a yeast protein histidine kinase, and since then several reports on histidine protein kinase activity in humans have been reported (5,6). Later a new 14-kDa phos- phatase that dephosphorylated phosphohistidine, but neither phosphoserine, -threonine, nor -tyrosine, was Correspondence: Pia Ek, Department of Medical Biochemistry and Microbiology, Uppsala University, Sweden. E-mail: Pia.Ek@imbim.uu.se (Received 11 November 2008; accepted 19 November 2008) ISSN 0300-9734 print/ISSN 1502-4725 online # 2009 Informa UK Ltd. (Informa Healthcare, Taylor & Francis AS) DOI: 10.1080/03009730802642337