Abstract
Phosducin is a cytosolic protein predominantly expressed in the retina
and the pineal gland that can interact with the betagamma subunits
of guanine nucleotide binding proteins (G proteins) and thereby may
regulate transmembrane signaling. A cDNA encoding a phosducin-like
protein (PhLP) has recently been isolated from rat brain Miles,
M. F., Barhite, S., Sganga, M. & Elliott, M. (1993) Proc. Natl. Acad.
Sci. USA 90, 10831-10835. Here we report the expression of PhLP in
Escherichia coli and its purification. Recombinant purified PUP inhibited
multiple effects of G-protein betagamma subunits. First, it inhibited
the betagamma-subunit-dependent ADP-ribosylation of purified alpha(o)
by pertussis toxin. Second, it inhibited the GTPase activity of purified
G(o). The IC50 value of PhLP in the latter assay was 89 nM, whereas
phosducin caused half-maximal inhibition at 17 nM. And finally, PhLP
antagonized the enhancement of rhodopsin phosphorylation by purified
betagamma subunits. The N terminus of PhLP shows no similarity to
the much longer N terminus of phosducin, the region shown to be critical
for phosducin-betagamma-subunit interactions. Therefore, PhLP appears
to bind to G-protein betagamma subunits by an as yet unknown mode
of interaction and may represent an endogenous regulator of G-protein
function.
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