PTX substrate with an a subunit of

Purification of GTP binding and hydrolyzing activity from bovine brain led to the isolation of a G protein with an a subunit of Mr 39000, that is a substrate for PTX [73-75]. It co-purifies with another PTX substrate of much lower abundance, also an afiy heterotrimer, but with an a subunit of M, 41000. The G protein with a of 39 kDa was termed G and its a subunit a or a39. The subscript o was meant to denote other , to distinguish it from the PTX substrate with 04 . The functional role of G is unknown. The protein with composition a4l/3y was termed Gj, because the GTP S-activated a subunit inhibited adenylyl cyclase activity in Gs defective cyc cells [76]. Isolated G did not have this effect [75]. Thus, the cyc assay identifies a4 Py of bovine brain but not a39/3y as a possible Gj. 

G and a41 Gj have also been purified from rat [77] and porcine [78] brain and shown to be distinct from each other. Both bovine and rat brain G or Gj were shown in reconstitution studies to interact with muscarinic receptors [79-81], by increasing the binding affinity for muscarinic agonists [80,81]. Platelet a2-adrenergic receptors [82] stimulate the GTPase activity of the bovine G and Gj proteins in 

Isolated G0 and Gj from rat brain also reconstitute with identical potency coupling of the chemotactic peptide (fMLP) receptor to a phosphoinositol bisphos-phate-specific phospholipase C in PTX-treated membranes of HL-60 cells. This identifies both G proteins as stimulators of phospholipase C, i.e., as Gp-type molecules as opposed to Grtype, suggesting that they have similar functions in cell regulation [84]. 

The disparity in these conclusions may reside in differences between bovine Gj and porcine Gs not detectable by simple SDS-PAGE migration. Indeed, a cDNA encoding bovine brain aj [86] and a cDNA encoding porcine brain a [87] show significant differences in amino acid composition in spite of large stretches of identity (see below). 

---