Crystal Structure at 2.4 Å Resolution of the Complex of Transducin βγ and Its Regulator, Phosducin

Abstract

The crystal structure of transducin's betagamma subunits complexed with phosducin, which regulates Gtbetagamma activity, has been solved to 2.4 angstroms resolution. Phosducin has two domains that wrap around Gtbetagamma to form an extensive interface. The N-terminal domain binds loops on the "top" Gtbeta surface, overlapping the Gtalpha binding surface, explaining how phosducin blocks Gtbetagamma's interaction with Gtalpha. The C-terminal domain shows structural homology to thioredoxin and binds the outer strands of Gtbeta's seventh and first blades in a manner likely to disrupt Gtbetagamma's normal orientation relative to the membrane and receptor. Phosducin's Ser-73, which when phosphorylated inhibits phosducin's function, points away from Gtbetagamma, toward a large flexible loop. Thus phosphorylation is not likely to affect the interface directly, but rather indirectly through an induced conformational change.

Function of phosducin-like proteins in G protein signaling and chaperone-assisted protein folding

“…This was found to be the case, both G t and Pdc are expressed at ∼ 1 copy per 10 rhodopsins [19,35]. Second, the structure of the Pdc-G t βγ complex, solved by X-ray crystallography, was consistent with a G t βγ sequestration function for Pdc [36,37]. The structure showed that Pdc bound the same face of G t βγ as G t α, with a great degree of overlap between the interaction surfaces.…”

“…This face of Gβγ also constitutes a major interaction site for other Gβγ effectors, including G protein receptor kinase 2 (GRK2), phospholipase Cβ (PLCβ), adenylyl cyclase 2 (AC2), GIRK K + channels and N-type Ca 2+ channels [38]. In addition to the effector binding surface, Pdc also occluded the membrane binding surface of G t βγ and buried the C-terminal farnesyl group of G t γ in a cleft of G t β created by Pdc binding [36,37,39]. These observations were consistent with the finding that Pdc formed a soluble complex with G t βγ by blocking its binding to rod disc membranes [14].…”

The protein disulphide-isomerase family: unravelling a string of folds

“…This includes the C-terminal domain of the transducin regulator phosducin, and the N-terminal domain of HORF6, a human peroxidase enzyme of the novel peroxiredoxin (Prx) class. [173,174].…”

Effects of two Gβγ-binding proteins – N-terminally truncated phosducin and β-adrenergic receptor kinase C terminus (βARKct) – in heart failure

“…The different effects on cAMP formation may be because of the capacity of phosducin, but not of bARKct, to bind to Gsa 13 and inhibit GDP release from Gsa, the rate-limiting step of G protein activation, or to its higher binding affinity for Gbg compared to bARKct. 15,17 An alternative explanation would be a specific bARKdirected dominant-negative effect of bARKct, but not of phosducin.…”

“…Gbg proteins exert a modulatory effect on phospholipase C, potassium channels, and other second messenger systems 11 and Gbg-mediated effects are inhibited by bARKct. 12 Therefore, we compared overexpression of bARKct with that of phosducin, another Gbg-binding protein, which also translocates upon receptor activation from the cytosol to the cell membrane, binds Gbg subunits with high affinity 13 and inhibits Gbg-mediated signaling. 14 Similar to bARKct, phosducin can compete with bARK for Gbg subunits and thereby impair bARKmediated phosphorylation of receptors.…”