Effect of deletion of the major brain G-protein ? subunit (?o) on coordination of G-protein subunits and on adenylyl cyclase activity

Heterotrimeric G-proteins, composed of α and βγ subunits, transmit signals from cell-surface receptors to cellular effectors and ion channels. Cellular responses to receptor agonists depend on not only the type and amount of G-protein subunits expressed but also the ratio of α and βγ subunits. Thus far, little is known about how the amounts of α and βγ subunits are coordinated. Targeted disruption of the αo gene leads to loss of both isoforms of αo, the most abundant α subunit in the brain. We demonstrate that loss of αo protein in the brain is accompanied by a reduction of β protein to 32 ± 2% (n = 4) of wild type. Sucrose density gradient experiments show that all of the βγ remaining in the brains of αo−/− mice sediments as a heterotrimer (s20,w = 4.4 S, n = 2), with no detectable free α or βγ subunits. Thus, the level of the remaining βγ subunits matches that of the remaining α subunits. Protein levels of α subunits other than αo are unchanged, suggesting that they are controlled independently. Coordination of βγ to α occurs posttranscriptionally because the mRNA level of the predominant β1 subtype in the brains of αo−/− mice was unchanged. Adenylyl cyclase can be positively or negatively regulated by βγ. Because the level of other α subunits is unchanged and αo itself has little or no effect on adenylyl cyclase, we could examine how a large change in the level of βγ affects this enzyme. Surprisingly, we could not detect any difference in the adenylyl cyclase activity between brain membranes from wild-type and αo−/− mice. We propose that αo and its associated βγ are sequestered in a distinct pool of membranes that does not contribute to the regulation of adenylyl cyclase. J. Neurosci. Res. 54:263–272, 1998. © 1998 Wiley-Liss, Inc.