Abstract
Heterotrimeric G proteins are molecular switches that regulate numerous signaling pathways involved in cellular physiology. This characteristic is achieved by the adoption of two principal states: an inactive, GDP bound state and an active, GTP bound state. Under basal conditions, G proteins exist in the inactive, GDP bound state; thus, nucleotide exchange is crucial to the onset of signaling. Despite our understanding of G protein signaling pathways, the mechanism of nucleotide exchange remains elusive. We employed phage display technology to identify nucleotide state-dependent Galpha binding peptides. Herein, we report a GDP-selective Galpha binding peptide, KB-752, that enhances spontaneous nucleotide exchange of Galpha(i) subunits. Structural determination of the Galpha(i1)/peptide complex reveals unique changes in the Galpha switch regions predicted to enhance nucleotide exchange by creating a GDP dissociation route. Our results cast light onto a potential mechanism by which Galpha subunits adopt a conformation suitable for nucleotide exchange.