Specificity of G alpha(q) and G alpha(11) gene expression in platelets and erythrocytes - Expressions of cellular differentiation and species differences

G alpha(q) and G alpha(11), members of the G(q) family of G-proteins, transduce signals from receptors to the beta isoenzymes of phosphatidylinositol-specific phospholipase C (PI-PLC). The receptor specificity of these alpha subunits is unknown. G alpha(q) and G alpha(11) are ubiquitously expressed in tissues; however, there have been conflicting reports of the presence or absence of G alpha(11) protein in haematopoietic cells. Platelet thromboxane A(2)/prostaglandin H-2 (TXA(2)/PGH(2)) receptors activate PI-PLC via G alpha(q), but the role of G alpha(11) is uncertain. To define their roles in platelet activation we studied G alpha(q) and G alpha(11) gene expression by immunotransfer blotting and by reverse transcription of mRNA followed by PCR (RT-PCR) and direct sequencing. An antiserum specific for mouse G alpha(11), failed to identify G alpha(11) in dog or human platelets or in dog liver, a tissue known to contain G alpha(11). RT-PCR performed with gene-specific primers demonstrated G alpha(q) mRNA, but not G alpha(11) mRNA, in normal human and mouse platelets and; in thromboxane-sensitive and thromboxane-insensitive dog platelets. Studies of mouse and dog liver and human retina confirmed that the cDNA, primers and probes used could amplify and recognize G alpha(11) in other tissues. However, species-specific oligonucleotide primers and probes were essential to demonstrate G alpha(11), but not G alpha(q), mRNA. Compared with mouse cDNA, dog and human G alpha(11) cDNA had twice as many nucleotide substitutions (approx. 12% compared with approx. 6%) as G alpha(q). G alpha(q) mRNA was also found in mature erythrocytes but G alpha(11) mRNA was not identified, whereas both G alpha(q) and G alpha(11) mRNAs were found in bone marrow stem cells. Therefore G alpha(11) gene expression in haematopoietic cells is linked with cellular differentiation. The lack of G alpha(11) indicates that signal transduction from platelet TXA(2)/PGH(2) receptors to PI-PLC occurs via G alpha(q), and that G alpha(11) deficiency,is not responsible for defective activation of PI-PLC in thromboxane-insensitive dog platelets. Despite the high degree of similarity that exists between G alpha(q) and G alpha(11), significantly greater species-specific variation in nucleotide sequence is present in G alpha(11) than in G alpha(q). Cellular specificity and species specificity are important characteristics of these G(q) family G-proteins.