Effect of deletion of the major brain G‐protein α subunit (α 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 α o gene leads to loss of both isoforms of α o , the most abundant α subunit in the brain. We demonstrate that loss of α o 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 α o −/− mice sediments as a heterotrimer (s 20,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 α o are unchanged, suggesting that they are controlled independently. Coordination of βγ to α occurs posttranscriptionally because the mRNA level of the predominant β 1 subtype in the brains of α o −/− mice was unchanged. Adenylyl cyclase can be positively or negatively regulated by βγ. Because the level of other α subunits is unchanged and α o 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 α o −/− mice. We propose that α o 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.