Immunohistochemical localization of PHPT1 69 Figure 2. Immunohistochemical staining for PHPT1. A: A section of adult mouse kidney shows PHPT1 expression in the distal convoluted tubules (arrowhead) but not in the glomeruli (arrow) and a weak expression in the proximal convoluted tubule. B: PHPT1 is expressed in the Henle’s loops of adult kidney. C and D: Sections of seminiferous tubule of adult mouse testis. Arrows in C point to the spermatogonium of seminiferous tubule. D: An absorption test shows that PHPT1 signals were abolished in the mouse testis. E: PHPT1 is expressed in the epithelium of interlobular duct of pancreas (arrow). F: PHPT1 is expressed in the epithelium of bronchiole (arrow). Arrowheads point at macrophages in alveoli. Ampliﬁcations were 400 for A F. simple, squamous epithelial cells, such as endothe- lium and alveolar epithelium, which were all nega- tive. Developing muscle also expressed PHPT1, such as the heart muscle (Figure 1E) and gut smooth muscle at embryonal stage E14.5, but a relatively lower expression level was found in the correspond- ing adult tissues. However, differences in the expression level be- tween mouse and human were also found. Thus in the bile duct epithelium PHPT1 was more strongly expressed in mouse than in human tissue, while the opposite was the case in pancreatic acinar cells (Table I). In the adrenal gland PHPT1 was only found in cells of the zona glomerulosa in mouse but was homogenously expressed throughout the cortex in human tissue. Different expression patterns between the species were also seen for spermatogonia of testis and the epithelium of proximal convoluted tubules of kidney, as outlined above (Table I). Discussion In two independent studies (7,8) a 14-kDa phos- phohistidine phosphatase (PHPT1) was isolated and characterized. Later studies on the enzyme have suggested that its substrates include a subunit of heterotrimeric G proteins (Gb) (9) and ATP-citrate lyase (10). However, as the turn-over rate for these substrates seems to be much lower than for the phosphohistidine peptide, used by Ek et al. (7), the identification of true physiological substrates may