Signal transducing membrane complexes of photoreceptor outer segments

Abstract

Signal transduction in outer segments of vertebrate photoreceptors is mediated by a series of reactions among multiple polypeptides that form protein-protein complexes within or on the surface of the disk and plasma membranes. The individual components in the activation reactions include the photon receptor rhodopsin and the products of its absorption of light, the three subunits of the G protein, transducin, the four subunits of the cGMP phosphodiesterase, PDE6 and the four subunits of the cGMP-gated cation channel. Recovery involves membrane complexes with additional polypeptides including the Na(+)/Ca(2+), K(+) exchanger, NCKX2, rhodopsin kinases RK1 and RK7, arrestin, guanylate cyclases, guanylate cyclase activating proteins, GCAP1 and GCAP2, and the GTPase accelerating complex of RGS9-1, G(beta5L), and membrane anchor R9AP. Modes of membrane binding by these polypeptides include transmembrane helices, fatty acyl or isoprenyl modifications, polar interactions with lipid head groups, non-polar interactions of hydrophobic side chains with lipid hydrocarbon phase, and both polar and non-polar protein-protein interactions. In the course of signal transduction, complexes among these polypeptides form and dissociate, and undergo structural rearrangements that are coupled to their interactions with and catalysis of reactions by small molecules and ions, including guanine nucleotides, ATP, Ca(2+), Mg(2+), and lipids. The substantial progress that has been made in understanding the composition and function of these complexes is reviewed, along with the more preliminary state of our understanding of the structures of these complexes and the challenges and opportunities that present themselves for deepening our understanding of these complexes, and how they work together to convert a light signal into an electrical signal.

Figure 1

Proposed model for first complex formed in photoactivation. Following absorption of light and photoisomerization by one subunit of the rhodopsin dimer, a heterodimer of rhodopsin and Metarhodopsin II (R*) is formed, which quickly complexes with the G protein, transducin, in its heterotrimeric form. A conformational change within G_αt allows release of bound GDP, which then allows binding of GTP and dissociation of activated G_αt–GTP. The representation of the complex is purely schematic, as only indirect low resolution information is available on the structures of the actual complexes. The major contacts with R* are provided by G_αt, with both its carboxyl and fatty acylated amino termini known to be involved. G_βγ are also important for R* binding, with potential interactions with the membrane hydrocarbon phase provided by the farnesyl group on G_γ. For making the structure images shown, PDB files 1U19.pdb(Okada, et al., 2004) and 1GOT.pdb(Lambright, et al., 1996) were used with UCSF Chimera (Meng, et al., 2006, Pettersen, et al., 2004).

Figure 2

A multi-subunit complex essential for normal photoresponse recovery kinetics. Sub-second GTP hydrolysis by activated G_αt–GTP is catalyzed by the GTPase accelerating complex of RGS9-1, G_β5L, and single-pass trans-membrane anchor protein, R9AP. Indirect evidence suggests that formation of this complex occurs while G_αt–GTP is bound to its effector PDE6, largely through interactions with PDE6_γ, whose C-terminal fragment, PDE6_γ’, (Slep et al., 2001) is shown in red space-filling representation. For making the structure images shown, the RGS domains from PDB files FQJ.pdb(Slep et al., 2001) and 2PBI(Cheever et al., 2008) were ligned in UCSF Chimera (Meng et al., 2006, Pettersen et al., 2004) and the resulting assembly of models positioned next to a model of holo PDE6, based on unpublished cryo-electron microscopy data kindly provided by Dr. Zhixian Zhang. The schematic representation of R9AP was loosely derived from a model presented previously(Cheever et al., 2008) The spatial relationships of PDE6_γ to holo PDE6, and of all of the polypeptides with respect to the membrane are not intended to be accurate; however, the attachment of the complex to the membrane via insertion of the isoprenyl tails of PDE6 and of the transmembrane segment of R9AP into the membrane is based on substantial biochemical evidence.