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Departments of 1 Biological Chemistry, 2 Immunology, and 3 Biological Regulation, The Weizmann Institute of Science, Rehovot 76100, Israel
Recent findings have suggested the
involvement of protein phosphorylation in the regulation of the
epithelial Na+ channel (ENaC). This study reports the in
vitro phosphorylation of the COOH termini of ENaC subunits expressed as
glutathione S-transferase fusion proteins. Channel subunits
were specifically phosphorylated by kinase-enriched cytosolic fractions
derived from rat colon. The phosphorylation observed was not mediated by the serum- and glucocorticoid-regulated kinase sgk. For
the
-subunit, phosphorylation occurred on a single, well-conserved threonine residue located in the immediate vicinity of the PY motif
(T630). The analogous residue on
(S620) was phosphorylated as well.
The possible role of
T630 and
S620 in channel function was
studied in Xenopus laevis oocytes. Mutating these residues to alanine had no effect on the basal channel-mediated current. They
do, however, inhibit the sgk-induced increase in channel activity but only in oocytes that were preincubated in low
Na+ and had a high basal Na+ current. Thus
mutating
T630 or
S620 may limit the maximal channel activity
achieved by a combination of sgk and low Na+.
epithelial sodium channel; serum- and glucocorticoid-dependent kinase; protein kinase; Nedd4; channel phosphorylation
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