G Protein β Subunit Types Differentially Interact with a Muscarinic Receptor but Not Adenylyl Cyclase Type II or Phospholipase C-β2/3

Abstract

In comparison with the ␣ subunit of G proteins, the role of the ␤ subunit in signaling is less well understood. During the regulation of effectors by the ␤␥ complex, it is known that the ␤ subunit contacts effectors directly, whereas the role of the ␤ subunit is undefined in receptor-G protein interaction. Among the five G protein ␤ subunits known, the ␤ 4 subunit type is the least studied. We compared the ability of ␤␥ complexes containing ␤ 4 and the well characterized ␤ 1 to stimulate three different effectors: phospholipase C-␤2, phospholipase C-␤3, and adenylyl cyclase type II. ␤ 4 ␥ 2 and ␤ 1 ␥ 2 activated all three of these effectors with equal efficacy. However, nucleotide exchange in a G protein constituting ␣ o ␤ 4 ␥ 2 was stimulated significantly more by the M2 muscarinic receptor compared with ␣ o ␤ 1 ␥ 2 . Because ␣ o forms heterotrimers with ␤ 4 ␥ 2 and ␤ 1 ␥ 2 equally well, these results show that the ␤ subunit type plays a direct role in the receptor activation of a G protein.The G protein ␤␥ complex regulates the activity of a diverse set of effectors, including phospholipases, adenylyl cyclases, and ion channels (1). There is evidence that the ␤ subunit in the complex interacts directly with effectors (2-5). There are five ␤ subtypes (␤ 1 -␤ 5 ) as well as an alternatively spliced version of ␤ 5 (known as ␤ 5 -long) (6 -11). ␤ 1 -␤ 4 share over 80% identity with one another, whereas ␤ 5 shares only ϳ50% identity with the other ␤ subunits (12). The divergence between ␤ 5 and the other ␤ subunits is consistent with the functional differences between ␤ 5 and ␤ 1 observed in effector regulation in a variety of systems (4,13,14). The high sequence similarity of ␤ 1 -␤ 4 suggests that their functions are conserved. Although some experiments indicated little difference in effector modulating capability among these ␤ subunit types, other experiments suggest otherwise. The G protein-coupled receptor kinase GRK3 binds ␤␥ complexes consisting of ␤ 1 , ␤ 2 , and ␤ 3 , but only ␤ 1 and ␤ 2 bind to the related kinase GRK2 (15). Other results indicate the selective mediation of cross-talk between G proteins and protein kinase C modulation of N-type channels by the ␤ 1 subunit type (16).Experiments focusing on the specific role of individual G protein subunit types have provided evidence for a certain level of selectivity in the interaction of ␣ subunit types with receptors (17). Evidence for similar selectivity of interaction between ␥ subunit types and receptors also exists (18 -20). In contrast there is limited evidence for ␤ subunit type selectivity in receptor interaction. Whole-cell experiments using antisense oligonucleotides directed against specific ␤ subunit cDNAs selectively disrupted signaling from particular receptors (21). Although the selective interaction of ␤ subunit types with receptors could give rise to this result, such selectivity has not been shown so far.Among the five ␤ subunits, ␤ 4 is the least studied. Its role in effector regulation and receptor interaction remains unclear. To exam...

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