A reaction-diffusion coupled system with multiple steady states.

In two intact cell systems in which GTP-binding protein (G) activity is initiated by the presence of agonist-bound receptors (R), it has been demonstrated that the rate of G activation is influenced by the rate of turnover of agonist occupancy among the receptor population. G activity is reduced when a low concentration of agonist-occupied receptors comprised by low fractional occupancy of a large receptor population is replaced by the presence of the same concentration of 100%-occupied receptors. This effect has been proposed to be due to a time interval of interaction between R and G (an encounter) that is long compared to the time of a single collection between R and G and long compared to the lifetime of an agonist-receptor complex. In a recent simulation study of R-G interaction via diffusion, the effect of agonist occupancy turnover was observed but it was assumed that long encounters were not operative. In this study, encounter intervals in simulations of R-G interaction by simple diffusion were measured in order to address that difference. The results demonstrate that relatively long encounters comprised of multiple, separate collisions are an inherent part of R-G interaction as modelled by diffusion. The implications for further implementation of simulation studies of R-G interaction are discussed.

Diffusion↗

PubMed1996

Effect of lipid molecular structure and gramicidin A on the core of lipid vesicle bilayers. A time-resolved fluorescence depolarization study.

We have investigated the molecular orientational order and reorientational dynamics of the fluorescent probe 1,6-diphenyl-1,3,5-hexatriene (DPH) in the core of the membrane bilayer. Vesicles of lipids of varying unsaturation and headgroup (POPC, DOPC, DLPC, DLLPC, EGGPG, DOPG, DGDG, and SQDG) were studied using the time-resolved fluorescence anisotropy of DPH. Generally, values of the second order parameter for DPH are found to be very small. However, this should not be interpreted as DPH having low orientational order as witnessed by large values of the next relevant order parameter . This implies considerable transverse populations of DPH molecules within the bilayer. In phosphatidylcholines with an acyl chain of 18 carbon atoms, the value of for DPH decreases with increasing lipid unsaturation and even attains negative values. No effect of the lipid headgroup on the order and dynamics of DPH is detected. Furthermore, we study the peptide-lipid interaction of the hydrophobic antibiotic gramicidin A (gA) in DOPC vesicles using DPH. The nonchannel conformation has an ordering effect on DPH in the bilayer core, which the channel confirmation lacks. This can be understood in terms of the geometrical shape of the gA dimer, as shown previously with the probes TMA-DPH and DPHPC [Muller, J. M., et al. (1995) Biochemistry 34, 3092]. We find that for DPH data the conventional Brownian rotational diffusion (BRD) model and the compound motion model (CMM) give equivalent fits. In this respect, DPH differs from TMA-DPH and DPHPC, for which probes only the CMM allowed a consistent interpretation of the molecular orientation.

Diffusion↗