科研之友
微信
新浪微博
Facebook
分享链接
摘要
The epithelial sodium channel (ENaC) is probably a heterotrimer with three well characterized subunits (alpha beta gamma). In humans an additional delta-subunit (delta-hENaC) exists but little is known about its function. Using the Xenopus laevis oocyte expression system, we compared the functional properties of alpha beta gamma- and delta beta gamma-hENaC and investigated whether delta beta gamma-hENaC can be proteolytically activated. The amiloride-sensitive ENaC whole-cell current (Delta I-ami) was about 11-fold larger in oocytes expressing delta beta gamma-hENaC than in oocytes expressing alpha beta gamma-hENaC. The 2-fold larger single-channel Na conductance of delta beta gamma-hENaC cannot explain this difference. Using a chemiluminescence assay, we demonstrated that an increased channel surface expression is also not the cause. Thus, overall channel activity of delta beta gamma-hENaC must be higher than that of alpha beta gamma-hENaC. Experiments exploiting the properties of the known beta S520C mutant ENaC confirmed this conclusion. Moreover, chymotrypsin had a reduced stimulatory effect on delta beta gamma-hENaC whole-cell currents compared with its effect on alpha beta gamma-hENaC whole-cell currents (2-fold versus 5-fold). This suggests that the cell surface pool of so-called near-silent channels that can be proteolytically activated is smaller for delta beta gamma-hENaC than for alpha beta gamma-hENaC. Proteolytic activation of delta beta gamma-hENaC was associated with the appearance of a delta-hENaC cleavage product at the cell surface. Finally, we demonstrated that a short inhibitory 13-mer peptide corresponding to a region of the extracellular loop of human alpha-ENaC inhibited Delta I-ami in oocytes expressing alpha beta gamma-hENaC but not in those expressing delta beta gamma-hENaC. We conclude that the delta-subunit of ENaC alters proteolytic channel activation and enhances base-line channel activity.
- 出版日期2009-10-16
