The PP-motif in luminal loop 2 of ZnT transporters plays a pivotal role in TNAP activation.

Secretory and membrane-bound zinc-requiring enzymes are thought to be activated by binding zinc in the early secretory pathway. One such enzyme, tissue-non-specific alkaline phosphatase (TNAP), is activated through a two-step mechanism, via protein stabilization and subsequent enzyme activation through metalation, by ZnT5-ZnT6 heterodimers or ZnT7 homodimers. However, little is known about the molecular basis underlying the activation process. In the present study, we found that the di-proline motif (PP-motif) in luminal loop 2 of ZnT5 and ZnT7 is important for TNAP activation. TNAP activity was significantly reduced in cells lacking ZnT5-ZnT6 heterodimers and ZnT7 homodimers [triple knockout (TKO) cells]. The decreased TNAP activity was restored by expressing hZnT5 with hZnT6 or hZnT7, but significantly less so (almost 90% less) by expressing mutants thereof in which the PP-motif was mutated to alanine (PP-AA). In TKO cells, overexpressed hTNAP was not completely activated, and it was converted less efficiently into the holo form by expressing a PP-AA mutant of hZnT5 with hZnT6, whose defects were not restored by zinc supplementation. The zinc transport activity of hZnT7 was not significantly impaired by the PP-AA mutation, indicating that the PP-motif is involved in the TNAP maturation process, although it does not control zinc transport activity. The PP-motif is highly conserved in ZnT5 and ZnT7 orthologues, and its importance for TNAP activation is conserved in the Caenorhabditis elegans hZnT5 orthologue CDF5. These results provide novel molecular insights into the TNAP activation process in the early secretory pathway.

Overview of Inherited Zinc Deficiency in Infants and Children.

Zinc nutrition is of special practical importance in infants and children. Poor zinc absorption causes zinc deficiency, which leads to a broad range of consequences such as alopecia, diarrhea, skin lesions, taste disorders, loss of appetite, impaired immune function and neuropsychiatric changes and growth retardation, thus potentially threatening life in infants and children. In addition to dietary zinc deficiency, inherited zinc deficiency, which rarely occurs, is found during the infant stage and early childhood. Recent molecular genetic studies have identified responsible genes for two inherited zinc deficiency disorders, acrodermatitis enteropathica (AE) and transient neonatal zinc deficiency (TNZD), clarifying the pathological mechanisms. Both of these zinc deficiencies are caused by mutations of zinc transporters, although the mechanisms are completely different. AE is an autosomal recessive disorder caused by mutations of the ZIP4 gene, consequently resulting in defective absorption of zinc in the small intestine. In contrast, TNZD is a disorder caused by mutations of the ZnT2 gene, which results in low zinc breast milk in the mother, consequently causing zinc deficiency in the breast-fed infant. In both cases, zinc deficiency symptoms are ameliorated by a daily oral zinc supplementation for the patients. Zinc is definitely one of the key factors for the healthy growth of infants and children, and thus zinc nutrition should receive much attention.