The Phosducin-Like Protein PhLP1 Is Essential for Gßgamma Dimer Formation in Dictyostelium discoideum

Phosducin proteins are known to inhibit G protein-mediated signaling by sequestering G beta gamma subunits. However, Dictyostelium discoideum cells lacking the phosducin-like protein PhLP1 display defective rather than enhanced G protein signaling. Here we show that green fluorescent protein (GFP)-tagged G beta (GFP-G beta) and GFP-G gamma subunits exhibit drastically reduced steady-state levels and are absent from the plasma membrane in phlp1(-) cells. Triton X-114 partitioning suggests that lipid attachment to GFP-G gamma occurs in wild-type cells but not in phlpI(-) and g beta(-) cells. Moreover, Goy dimers could not be detected in vitro in coimmunoprecipitation assays with phlp1(-) cell lysates. Accordingly, in vivo diffusion measurements using fluorescence correlation spectroscopy showed that while GFP-G gamma proteins are present in a complex in wild-type cells, they are free in phlp1(-) and g beta(-) cells. Collectively, our data strongly suggest the absence of G beta gamma dimer formation in Dictyostelium cells lacking PhLP1. We propose that PhLP1 serves as a cochaperone assisting the assembly of Go and G gamma into a functional G beta gamma complex. Thus, phosducin family proteins may fulfill hitherto unsuspected biosynthetic functions.