The human genome encodes at least three non‐allellic G proteins with αi‐type subunits

Abstract
The amino acid sequence and composition of α‐subunits of signal transducing G proteins of the same kind appear to vary by no more than 2% from species to species. Here we isolated a human liver cDNA using an oligonucleotide complementary to the sequences encoding the pertussis toxin (PTX) ADP‐ribosylation site of the α‐subunit of the rat brain G protein called Gi. Its open reading frame characterizes it as an αi‐type cDNA—as opposed to αo‐type—but predicts an amino acid composition that differs by 7% and 14%, respectively, from two other human αi‐type molecules. Together with human brain αi (type‐1) and human monocyte αi (type‐2), the new human liver αi cDNA (type‐3) forms part of a family of αi molecules. Type‐3 αi cDNA hybridizes to a ∼ 3.6 kilobase long mRNA and type‐2 αi cDNA hybridizes to an mRNA species of ∼ 2.7 kilobases. This indicates that the human genome has at least three non‐allellic genes encoding non‐αo‐type PTX substrates and provides structural evidence for the hypothesis that distinct effector systems are regulated by similar but nevertheless distinct PTX substrates.