Evidence for ATP-ase activity of arrestin from bovine photoreceptors

AbstractIn vertebrate photoreceptors the soluble protein arrestin (45 kDa) is involved in controlling the light dependence activity of receptor proteins such as transducin or the cGMP-phosphodiesterase. Arrestin has further been identified as the retinal-S-antigen which is assumed to cause the autoimmune discase uveitis. In a first communication a binding of the nucleotide ATP to arrestin was described. In this subsequent study it is shown that arrestin is also able to hydrolyse ATP at a rate (5.1 ±0.3)·10−3 U/mg min with C12 = 93±5 nM and a Hill coefficient n = 1.8±0.1 at pH 7.2 and 20°C. These findings suggest a new insight into the process of regulating photoreceptor activity

Enhancing Egress Drills: Preparation and Assessment of Evacuee Performance

This article explores how egress drills-specifically those related to fire incidents-are currently used, their impact on safety levels, and the insights gained from them. It is suggested that neither the merits of egress drills are well understood, nor the impact on egress performance well characterized. In addition, the manner in which they are conducted varies both between and within regulatory jurisdictions. By investigating their strengths and limitations, this article suggests opportunities for their enhancement possibly through the use of other egress models to support and expand upon the benefits provided. It is by no means suggested that drills are not important to evacuation safety-only that their inconsistent use and the interpretation of the results produced may mean we (as researchers, practitioners, regulators, and stakeholders) are not getting the maximum benefit out of this important tool

On the Origin and the Signal-Shaping Mechanism of the Fast Photosignal in the Vertebrate Retina

Fast photosignals (FPS) with R(1) and R(2) components were measured in retinas of cattle, rat, and frog within a temperature range of 0° to 60°C. Except for temperatures near 0°C the signal rise of the R(1) component was determined by the duration of the exciting flash. The kinetics of the R(2) component and the meta transition of rhodopsin in the cattle and rat retina were compared. For the analysis of the FPS it is presupposed that the signal is produced by light-induced charges on the outer segment envelope membrane that spread onto the whole plasma membrane of the photoreceptor cell. To a good approximation, this mechanism can be described by a model circuit with two distinct capacitors. In this model, the charging capacitance of the pigmented outer segment envelope membrane and the capacitance of the receptor's nonpigmented plasma membrane are connected via the extra- and intracellular electrolyte resistances. The active charging is explained by two independent processes, both with exponential rise (R(1) and R(2)), that are due to charge displacements within the pigmented envelope membrane. The time constant τ(2) of the R(2) membrane charging process shows a strong temperature dependence that of the charge redistribution, τ(r), a weak one. In frog and cattle retinas the active charging is much slower within a large temperature range than the passive charge redistribution. From the two-capacitor model it follows for τ(r) « τ(2) that the rise of the R(2) component is determined by τ(r), whereas the decay is given by τ(2). For the rat retina, however, τ(2) approaches τ(r) at physiological temperatures and becomes <τ(r) above 45°C. In this temperature range where τ(2) ≈ τ(r), both processes affect rise and decay of the photosignal. The absolute values of τ(r) are in good accordance with the known electric parameters of the photoreceptors. At least in the cattle retina, the time constant τ(2) is identical with that of the slow component of the meta II formation. The strong temperature dependence of the meta transition time gives rise to the marked decrease of the R(2) amplitude with falling temperature. As the R(1) rise could not be fully time resolved the signal analysis does not yield the time constant τ(1) of the R(1) generating process. It could be established, however, within the whole temperature range that the decay of the R(1) component is determined by τ(r). Using an extended model that allows for membrane leakage, we show that in normal ringer solution the membrane time constant does not influence the signal time-course and amplitude

Lateral diffusion of lipids and glycophorin in solid phosphatidylcholine bilayers. The role of structural defects.

The lateral mobility of the lipid analog N-4-nitrobenzo-2-oxa-1,3 diazole phosphatidylethanolamine and of the integral protein glycophorin in giant dimyristoylphosphatidylcholine vesicles was studied by the photobleaching technique. Above the temperature of the chain-melting transition (Tm = 23 degrees C), the diffusion coefficient, Dp, of the protein [Dp = (4 +/- 2) X 10(-8) cm2/s at 30 degrees C] was within the experimental errors equal to the corresponding values DL of the lipid analog. In the P beta 1 phase the diffusion of lipid and glycophorin was studied as a function of the probe and the protein concentration. (a) At low lipid-probe content (cL less than 5 mmol/mol of total lipid), approximately 20% of the probe diffuses fast (D approximately equal to 10(-8) - 10(-9) cm2/s), while the mobility of the rest is strongly reduced (D less than 10(-10) cm2/s). At a higher concentration (cp approximately 20 mmol), all probe is immobilized (D less than 10(-10) cm2/s). (b) Incorporation of glycophorin up to cp = 0.4 mmol/mol of total lipid leads to a gradual increase of the fraction of mobile lipid probe due to the lateral-phase separation into a pure P beta 1 phase and a fraction of lipid that is fluidized by strong hydrophilic lipid-protein interaction. (c) The diffusion of the glycophorin molecules is characterized by a slow and a fast fraction. The latter increases with increasing protein content, which is again due to the lateral-phase separation caused by the hydrophilic lipid-protein interaction. The results are interpreted in terms of a fast transport along linear defects in the P beta 1 phase, which form quasi-fluid paths for a nearly one dimensional and thus very effective transport. Evidence for this interpretation of the diffusion measurements is provided by freeze-fracture electron microscopy