Inhibition of G-protein beta gamma-subunit functions by phosducin-like protein

Phosducin is a cytosolic protein predominantly expressed in the retina and the pineal gland that can interact with the beta gamma 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 PhLP inhibited multiple effects of G-protein beta gamma subunits, First, it inhibited the beta gamma-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 beta gamma 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-beta gamma-subunit Interactions, Therefore, PhLP appears to hind to G-protein beta gamma subunits by an as yet unknown mode of interaction and may represent an endogenous regulator of G-protein function.