Secretory and membrane bound, zinc-requiring enzymes are thought to be activated by binding zinc in the early secretory pathway. One such enzyme, tissue-nonspecific 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. Here, 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, over-expressed hTNAP was not completely activated, and it was converted less efficiently to 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 orthologs, and its importance for TNAP activation is conserved in the Caenorhabditis elegans hZnT5 ortholog CDF5. These results provide novel molecular insights into the TNAP activation process in the early secretory pathway.
- early secretory pathway
- zinc-requiring enzyme
- zinc transport
- ©2016 The Author(s)
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