Different β-subunits determine G-protein interaction with transmembrane receptors

Abstract

REGULATORY GTP-binding proteins (G proteins) are membrane-attached heterotrimers (α, β, γ) that mediate cellular responses to a wide variety of extracellular stimuli^1,2. They undergo a cycle of guanine-nucleotide exchange and GTP hydrolysis, during which they dissociate into αsubunit and βγ complex¹. The roles of G-protein αsubunits in these processes and for the specificity of signal transduction are largely established; the α- and γ-subunits are essential for receptor-induced G-protein activation and seem to be less diverse and less specific. Although the complementary DNAs for several β-subunits have been cloned^2,5–8, isolated sub-units have only been studied as βγ complexes^3,9–12. Functional differences have been ascribed to the γ-subunit on the basis of extensive sequence similarity among β-subunits and apparent heterogeneity in γ-subunit sequences^13,14.βγ complexes can interact directly or indirectly with different effectors^{10,11,15–20}. They seem to be interchangeable in their interaction with pertussis toxinsensitive α-subunits³, so we tested this by microinjecting antisense oligonucleotides into nuclei of a rat pituitary cell line to suppress the synthesis of individual β-subunits selectively. Here we show that two out of four subtypes of β-subunits tested (β₁ and β₃) are selectively involved in the signal transduction cascades from muscarinic M₄ (ref. 4) and somatostatin receptors, respectively, to voltage-dependent Ca²⁺ channels.