Rhodopsin acylation

A variety of proteins are acylated by formation of thioesters to cysteine and esters to serine and threonine. Acylation may serve either to anchor the proteins in membranes (e.g., rhodopsin Section 2.3.1) and the mannosidase of the Golgi, or to increase lipophilicity and thus enhance the solubilization of lipids being transported (e.g., the plasma apolipoproteins and milk globule proteins). Proteolipids with fatty acids esterified to threonine residues occur in the myelin sheath in nerves. 

Fig. 3. Schematic representation of lateral domains in a bilayer consisting of dil6 0 PC (dark ovals), di22 6 PC(striated ovals), cholesterol (small, light ovals) in a 7 3 3 ratio and rhodopsin (large gray ovals) at a 100 1 ratio of PC rhodopsin. Rliodopsin is in a cluster, highly enriched in 22 6n-3 acyl chains, whereas cholesterol is mainly associated with the saturated 16 0 acyl chains. The enrichment of di22 6 PC in the cluster around rhodopsin is enhanced about six times relative to the bulk concentration. The cluster extends about three layers around rhodopsin.

In visual signal transduction, activation proceeds from the receptor, rhodopsin, to the effector, phosphodiesterase (PDE), via the visual G protein, Gp Each Mil sequentially binds and activates up to 100 Gj, thus MII-Gj binding initiates the first stage of signal amplification in the visual pathway. Litman et al. (2001) have studied the phospholipid acyl chain dependence of the kinetics of formation of both the Mil conformation and the... 

Fig. 4. Examples of the effects of acyl chain composition (white bars) and cholesterol (gray bars) on A eq for the MI-MII equilibrium of photolyzed rhodopsin at 37°C. Higher values of correspond to higher equilibrium concentrations of Mil, the state of photolyzed rhodopsin that participates in visual signal transduction by binding the visual G protein.

Gibson NJ, Brown MF. Lipid headgroup and acyl chain composition modulate the MI-MII equilibrium of rhodopsin in recombinant membranes. Biochemistry 1993 32 2438-2454. 

Straume M, Litman BJ. Equilibrium and dynamic bilayer structural properties of unsaturated acyl chain phosphatidylcholine-cholesterol-rhodopsin recombinant vesicles and rod outer segment disk membranes as determined from higher order analysis of fluorescence anisotropy decay. Biochemistry 1988 27 7723-7733. 

Functionally, recoverin acts by binding to rhodopsin kinase and inhibits rhodopsin phosphorylation (88). It has also been observed to bind rod outer segment (ROS) membranes (89). The interaction with rhodopsin kinase does not require amino-terminal acylation, but binding to ROS membranes is acylation dependent, suggesting two distinct binding sites and/or binding modes on recoverin. 

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