Analysis of the T beta gamma-binding domain of MEKA/phosducin

The physiological roles of phosducin: from retinal function to stress-dependent hypertension

In the time since its discovery, phosducin's functions have been intensively studied both in vivo and in vitro. Phosducin's most important biochemical feature in in vitro studies is its binding to heterotrimeric G protein βγ-subunits. Data on phosducin's in vivo relevance, however, have only recently been published but expand the range of biological actions, as shown both in animal models as well as in human studies. This review gives an overview of different aspects of phosducin biology ranging from structure, phylogeny of phosducin family members, posttranscriptional modification, biochemical features, localization and levels of expression to its physiological functions. Special emphasis will be placed on phosducin's function in the regulation of blood pressure. In the second part of this article, findings concerning cardiovascular regulation and their clinical relevance will be discussed on the basis of recently published data from gene-targeted mouse models and human genetic studies.

The pharmacology of phosducin

The discovery of phosducin (Phd) in photoreceptor cells of the retina and the further identification of phosducin-like proteins (PhdLP) emphasizes the existence of a family of proteins characterized as cytosolic regulators of G protein functions. The individual members represent phosphoproteins with distinct tissue distributions whose highest concentrations were in the retina and the pineal gland, while lower levels were reported for tissues such as liver, spleen, striated muscle, and the brain. Several functions of Phd and PhdLP have been suggested, but their most important ability appears to be their high affinity sequestration with G betagamma subunits of heterotrimeric G proteins. This finding suggests that neutralization of G betagamma by Phd effectively impedes G protein-mediated signal transmission, since G alpha cannot reassemble with G betagamma to provide a functional G protein trimer (G alphabetagamma). Thus, it is the scavenger quality of Phd that is hypothesized to diminish intracellular communication simply by reducing the number of G proteins. An additional important function of Phd relates to the inhibition of G alpha subunits' inherent GTPase. The ability of Phd to directly bind G alpha subunits is probably of minor significance as the affinity between both proteins is low. In general, similar mechanisms have been reported for PhdLPs. In the majority of investigations concerning the interference of Phd with physiological mechanisms, the dark/light adaptation of retinal photoreceptor cells has been the most frequently studied aspect of Phd. More recently, Phd was associated with the adenylyl cyclase of olfactory cilia, as in the presence of the phosphoprotein an increased concentration of cAMP is observed. This finding is in line with the experimental outcome of permanent cell lines transfected to overexpress Phd, which exhibit sensitization to excitatory acting PGE(1), and isoproterenol, respectively. Furthermore, Phd was found to effectively slow down the mechanism of internalization of G protein-coupled opioid receptors. Pathophysiological processes associated with Phd were found for certain eye diseases. Experimental evidence suggests the development of retinal inflammation as a consequence of an autoimmunization process triggered by Phd or shorter fragments thereof. Thus, our present knowledge regarding the functions of members of the Phd family is limited currently to their control of G protein-mediated intracellular signal transmission, the process of endocytosis, and certain autoimmune diseases of the uvea and the pineal gland. However, recent information regarding the presence of certain members of the Phd family in the cell nucleus may bear new insights into the function of these compounds.

Functional roles of the two domains of phosducin and phosducin-like protein

Phosducin and phosducin-like protein regulate G protein signaling pathways by binding the betagamma subunit complex (Gbetagamma) and blocking Gbetagamma association with Galpha subunits, effector enzymes, or membranes. Both proteins are composed of two structurally independent domains, each constituting approximately half of the molecule. We investigated the functional roles of the two domains of phosducin and phosducin-like protein in binding retinal G(t)betagamma. Kinetic measurements using surface plasmon resonance showed that: 1) phosducin bound G(t)betagamma with a 2. 5-fold greater affinity than phosducin-like protein; 2) phosphorylation of phosducin decreased its affinity by 3-fold, principally as a result of a decrease in k(1); and 3) most of the free energy of binding comes from the N-terminal domain with a lesser contribution from the C-terminal domain. In assays measuring the association of G(t)betagamma with G(t)alpha and light-activated rhodopsin, both N-terminal domains inhibited binding while neither of the C-terminal domains had any effect. In assays measuring membrane binding of G(t)betagamma, both the N- and C-terminal domains inhibited membrane association, but much less effectively than the full-length proteins. This inhibition could only be described by models that included a change in G(t)betagamma to a conformation that did not bind the membrane. These models yielded a free energy change of +1.5 +/- 0.25 kcal/mol for the transition from the G(t)alpha-binding to the Pd-binding conformation of G(t)betagamma.

Functional analysis of Plp1 and Plp2, two homologues of phosducin in yeast

Mammalian phosducins are known to bind G protein betagamma subunits in vitro, and are postulated to regulate their signaling function in vivo. Here we describe two homologues of phosducin in yeast, called PLP1 and PLP2. Both gene products were cloned, expressed, and purified as glutathione S-transferase fusions. Of the two isoforms, Plp1 bound most preferentially to Gbetagamma. Binding was enhanced by pheromone stimulation and by the addition of GTPgammaS, conditions that favor dissociation of Gbetagamma from Galpha. Gene disruption mutants and gene overexpression plasmids were prepared and analyzed for changes in signaling and nonsignaling phenotypes. Haploid spore products bearing the plp2Delta mutant failed to grow, suggesting that PLP2 is an essential gene. Cell viability was not restored by a mutation in STE7 that blocks signaling downstream of the G protein. Haploid products bearing the plp1Delta mutant were viable and exhibited a 6-7% increase in pheromone-mediated gene induction. Cells overexpressing PLP1 or PLP2 exhibited a 70-80% decrease in gene induction but no change in pheromone-mediated growth arrest. These data indicate that phosducin can selectively regulate early signaling events following pheromone stimulation and has an essential role in cell growth independent of its regulatory role in cell signaling.

A molecular mechanism for the phosphorylation-dependent regulation of heterotrimeric G proteins by phosducin

Visual signal transduction is a nearly noise-free process that is exquisitely well regulated over a wide dynamic range of light intensity. A key component in dark/light adaptation is phosducin, a phosphorylatable protein that modulates the amount of transducin heterotrimer (Gt alpha beta gamma) available through sequestration of the beta gamma subunits (Gt beta gamma). The structure of the phosphophosducin/Gt beta gamma complex combined with mutational and biophysical analysis provides a stereochemical mechanism for the regulation of the phosducin-Gt beta gamma interaction. Phosphorylation of serine 73 causes an order-to-disorder transition of a 20-residue stretch, including the phosphorylation site, by disrupting a helix-capping motif. This transition disrupts phosducin's interface with Gt beta gamma, leading to the release of unencumbered Gt beta gamma, which reassociates with the membrane and Gt alpha to form a signaling-competent Gt alpha beta gamma heterotrimer.