plasma membrane

Figure 1. Phototransduction in the rod cell of the retina. Left: diagram of retinal rod cell, showing the direction of incoming light and cellular compartments of this highly differentiated neuron. The ROS, connected with the inner segment (RIS) through a narrow cilium, is densely packed with a stack of disk membranes. These internal vesicles contain an integral membrane protein, rhodopsin. Right: absorption of photon by rhodopsin's chromophore causes isomerization of ll-cw-retinylidene to all-trans-retinylidene. A sequence of protein-protein interactions between the G-protein-coupled receptor (rhodopsin, R), a G-protein (transducin, Gt), and an effector [a cGMP specific phosphodiesterase (PDE)] is the mechanism of visual transduction that allows conversion of the photon signal into the biochemical cascade of events. The signal is initially amplified on the level of the receptor, because hundreds of molecules of Gt interact with a single activated rhodopsin molecule, and on the level of the effector, as a consequence of its catalytic property. Light-activated rhodopsin (R*) is mobile through diffusion along the two-dimensional disk membrane plane and, when it encounters Gt, induces GDP release from the G-protein, and forms a transient nucleotide-free R**Gt complex. GTP, present in the ROS, dissociates this complex immediately by binding into the nucleotide-binding pocket of the Gta-subunit. Activated GTP-bound Gta is then capable of activating the effector by binding to its inhibitory subunits. The activated effector reduces the level of cGMP and, in turn, leads to the closure of cGMP-gated cation channels. A decreased influx of cations through the channel causes hyper-polarization of the plasma membrane, spreading the electrical signal to the synaptic terminal, and a subsequent decrease in the transmitter release (see text for details).

retina contains over 100 million photoreceptor cells [4] - highly differentiated post-mitotic neurons that consist of an inner and outer segment and an axonal part with its synaptic ending. In the retina of warm-blooded animals, ROS is connected via a thin, ~1.5 |im in diameter, cilium to the cell body.

Rhodopsin is embedded within the membranes of ROS, which are arranged in a long, closely spaced stack of approximately one thousand isolated disklike saccules, termed disks. Gt is associated with the cytoplasmic surface of the disk membranes. To provide an effective target for light, rhodopsin accounts for half of the dry weight of the disk membranes. Crucial for the function of R* is the high fluidity of the disk membrane. The high fluidity is a result of the large amounts of highly unsaturated (22:6«-3) acyl chains in its major phospholipids, phosphatidylcholine (PC), phosphatidylserine (PS) and phos-phatidylethanolamine (PE) [9]. A lack of polyunsaturated fatty acids causes abnormalities in visual function [10,11]. Also, the high fluidity and other properties of the membrane's special composition may be crucial for the anchoring of G-protein and effector to the membrane surface, and thus for proper signal transduction in the disk membrane. Understanding the role of membrane properties in signal transduction is, at the physicochemical level, still not well understood. However, with regard to photoreceptor function, there are two identified properties of membranes: the formation of the active intermediate and the transport of the hydrophobic retinal ligand.

The fluidity of the membrane enables fast lateral and rotatory diffusion of rhodopsin, with trajectories over the membrane surface in the order of one second. All protein-protein interactions, on which the G-protein coupled signal transmission relies, are localized on the surface of the disk membrane. Molecular recognition between these proteins is bound to active phases, which result from the intramolecular processes and the uptake of cofactors such as GTP (see [12]).

3.2.2 G-protein and the effector activation

In its inactive, GDP bound state (Gt'GDP), the heterotrimeric Gt holoprotein (Gta^y) is peripherally bound to the disk membrane by weak hydrophobic and ionic interactions [13-15]. The first step of nucleotide exchange catalysis is the collisional interaction between light-activated rhodopsin (R*) and Gt*GDP (step 1, Figure 1). This interaction triggers the release of GDP and subsequent formation of a stable R**Gt complex with an empty nucleotide binding site on the Gta-subunit (step 2). Binding of GTP to the Gta-subunit within the R**Gt complex enables a conformational change (step 3) that induces the release of active Gt'GTP (Gt*) from the receptor (step 4) and the (simultaneous or immeasurably delayed) separation of the a- and (3y-subunits (GtocGTP and Gt(3y). In vitro, activation is accompanied by an immediate (delay < 1 ms, [16]) dissociation of both Gta*GTP and Gt(3y from the disk membrane. The high rate of R*-catalyzed nucleotide exchange leads to the rapid accumulation of Gt*. The visual system utilizes the Ga-subunit to relay the signal to the effector. Active Gta'GTP, in turn, binds to the effector cGMP-specific PDE; this stoichiometric, non-catalytic interaction occurs within less than 5 ms [16]. The interaction keeps the PDE active, and hydrolysis of cGMP leads to the closure of several hundred cGMP-gated ion channels which control the flow of Na+ and Ca2+ ions into the photoreceptor ROS (reviewed in [17]). The resulting hyperpolarization of the rod plasma membrane inhibits the release of glutamate neurotransmitter at the synapse, which establishes the light signal that is transmitted to the brain via the nervus opticus [4].

The rising phase of the electrical response is dictated by the diffusional encounter between R* and Gt on the disk membrane. A 50% reduction of receptor density in the rod disks causes acceleration of both response onset and recovery of flash responses [18]. Nucleotide exchange catalysis in Gt by R* establishes a first step amplification, because one R* can activate several hundred G-proteins. This has been demonstrated for isolated disk membranes in vitro [12], but the exact number is not yet known in vivo.

3.2.3 Deactivation

To terminate signal transduction and to allow repeated and/or graded excitation of the cell, each single step of the transduction cascade must be properly deactivated. This happens via interactions with regulatory proteins. Rapid shut-off of active rhodopsin does not happen by thermal decay of the active conformation, but rather by concurrent interaction with rhodopsin kinase (RK) and eventually phosphorylation of the receptor. Phosphorylated rhodopsin enables interaction with arrestin and by this shut-off of the signal for the G-protein. Inactivation of Gt'GTP results from the hydrolysis of bound GTP in the nucleotide binding site of the Gta-subunit, while Gt is bound to the y-subunit of PDE. Other proteins regulate and accelerate this reaction [19]. To terminate the hyperpolarization of the rod cell membrane, the cGMP level in the cytoplasm must be restored by resynthesis of cGMP, which is regulated by a feedback mechanism. Since the Na+/Ca2+-K+ exchanger continues to extrude Ca2+ from the ROS, the concentration of intracellular Ca2+ decreases. This leads to activation of the Ca2+-binding proteins, the guanylate cyclase-activating proteins (GCAP1/GCAP2 and GCAP3), which in turn activate the enzyme guanylate cyclase. The resulting rise in cGMP causes the cGMP-gated channels to reopen and, consequently, causes the Na+/Ca2+ influx to terminate the hyperpolarization and decrease the guanylate cyclase activity by negative feedback inhibition (for details see [17,20,21]).

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