Resolution of phospholipid conformational heterogeneity in model membranes by spin-label EPR and frequency-domain fluorescence spectroscopy

The amide class herbicide 3,4-dichloropropionanilide (DCPA) alters the mobility of hydrocarbon chains in T-lymphocyte but not macrophage membranes

Previous studies in our laboratory have demonstrated that the lipophilic herbicide 3,4-dicholoropropionanilide (DCPA) adversely affects cytokine production by activated macrophages and T lymphocytes. The purpose of this study was to test the hypothesis that DCPA alters the mobility of plasma membrane lipid hydrocarbon chains, which interferes with normal T-lymphocyte activation and macrophage function. Electron spin reasonance (ESR) spectroscopy of stearic acid spin labels incorporated into each cell type was used to test the effects of DCPA on lipid hydrocarbon chain mobility in the absence and presence of specific agents that activate each cell type. The results indicated that DCPA treatment had no significant effect on hydrocarbon chain mobility in either cell type per se. However, for T lymphocytes, but not macrophages, DCPA treatment increased a small population of lipid molecules that exhibited reduced hydrocarbon chain mobility near the bilayer hydrocarbon core following cell stimulation. In contrast, there were no significant effects of DCPA on hydrocarbon chain mobility near the head group region of the bilayer for either cell type. The identity of this subpopulation of lipids and its motional properties could not be elucidated from these studies. Nevertheless, data show that DCPA alters the distribution of lipids in distinct motional environments in the membrane of activated T lymphocytes.

Phosphatidylethanolamine modulates Ca-ATPase function and dynamics

Phospholipids containing phosphoethanolamine (PE) headgroups within biological membranes have been suggested to be important with respect to the functional regulation of membrane proteins, including the Ca-ATPase in sarcoplasmic reticulum (SR). To investigate the role of PE headgroups in modulating the catalytic activity of the Ca-ATPase, we have reconstituted the Ca-ATPase into unilamellar liposomes containing defined amounts of dioleoylphosphatidylethanolamine (DOPE) and dioleoylphosphatidylcholine (DOPC). The enzymatic activity of the Ca-ATPase progressively increases upon incorporation of increasing amounts of PE into reconstituted vesicles, and approaches that characteristic of native SR membranes. To identify structural changes that correlate with enzyme activation, we have used frequency-domain phosphorescence spectroscopy to measure the rotational dynamics of erythrosin isothiocyanate covalently bound to Lys464 in the phosphorylation domain of the Ca-ATPase. Progressive increases in the rotational dynamics of the phosphorylation domain result from the incorporation of increasing amounts of DOPE, and correlate with enhanced enzymatic function. These results suggest that PE headgroups induce dynamic structural rearrangements involving the phosphorylation domain that modify the rates of nucleotide utilization. In contrast, no changes in the rotational dynamics of the lipid acyl chains are observed irrespective of the PE content. Therefore, the enhanced ATP hydrolytic activity associated with the incorporation of DOPE into these proteoliposomes is the result of specific noncovalent interactions involving PE phospholipid headgroups and the Ca-ATPase.

Enhanced rotational dynamics of the phosphorylation domain of the Ca-ATPase upon calcium activation

We have used labeling conditions that permit the specific and covalent attachment of erythrosin isothiocyanate (Er-ITC) to Lys464 within the phosphorylation domain of the Ca-ATPase in skeletal sarcoplasmic reticulum membranes. These labeling conditions do not interfere with high-affinity ATP binding, phosphoenzyme formation, or phosphoenzyme hydrolysis [Huang, S., Negash, S., and Squier, T. C. (1998) Biochemistry 37, 6949-6957]. Thus, we can use frequency-domain phosphorescence spectroscopy to measure the rotational dynamics of the Ca-ATPase stabilized in different enzymatic states corresponding to the absence of bound ligands (E), calcium activation (E x Ca2), the presence of bound nucleotide (E x ATP), and formation of phosphoenzyme (E-P). We resolve three rotational correlation times corresponding to (i) a large-amplitude domain motion of the phosphorylation domain (phi1 approximately 5 +/- 1 micros), (ii) overall protein rotational motion with respect to the membrane normal (phi2 approximately 50 +/- 10 micros), and (iii) the rotational motion of the SR vesicles (phi3 approximately 1.1 +/- 0.4 ms). No differences are observed in the rotational dynamics of E, E x ATP, or E-P, indicating that phosphoenzyme formation or nucleotide binding result in no global structural changes involving the phosphorylation domain. In contrast, calcium activation enhances the amplitude of motion of the phosphorylation domain. These observed calcium-dependent changes in rotational dynamics result from structural changes within a single Ca-ATPase polypeptide chain, since protein-protein interactions do not change upon calcium binding. Thus, calcium binding induces concerted domain motions within a single Ca-ATPase polypeptide chain that may play a critical role in facilitating substrate binding and utilization.

Phospholipid acyl chain rotational dynamics are independent of headgroup structure in unilamellar vesicles containing binary mixtures of dioleoyl-phosphatidylcholine and dioleoyl-phosphatidylethanolamine

We have examined relationships between phospholipid headgroup structure and acyl chain dynamics, and their respective roles in modulating the physical properties of biological membranes. Fluorescence lifetime and anisotropy measurements were used to assess structural changes involving the lipid acyl chains in homogeneous populations of small and large unilamellar vesicles containing binary mixtures of dioleoyl-phosphatidylcholine (PC) and dioleoyl-phosphatidylethanolamine (PE) in the liquid-crystalline (Lalpha) phase. These measurements involve three different fluorescent lipid analogs containing diphenylhexatriene (DPH) linked to either a trimethylamine moiety (i.e., TMA-DPH) or the sn-1 position of monostearoyl-phospholipids containing PC or PE headgroups (i.e., DPH-PC and DPH-PE). The average lifetimes, rotational correlation times, and order parameters associated with DPH-PC and DPH-PE are virtually identical, and are not affected by alterations in the PE content of the membrane. These results suggest that the average cross-sectional areas of the phospholipid acyl chains of DOPE and DOPC relative to the membrane normal are similar in these unilamellar vesicles. Since PC headgroups are larger than those of PE, differences in the relative orientation of the phosphocholine and phosphoethanolamine moieties relative to the membrane surface probably function to maintain optimal van der Waals contact interactions between acyl chains. On the other hand, the average lifetime associated with TMA-DPH, whose chromophoric group is near the membrane surface, increases with increasing PE content. The position of TMA-DPH relative to the membrane surface does not change, since the rotational dynamics of TMA-DPH are independent of the PE concentration. Therefore, alterations in the average lifetime of TMA-DPH results from polarity differences near the membrane surface at the level of the glycerol backbone. These results are discussed in terms of how differences in the average conformation of the glycerol backbones or phospholipid headgroups of PE and PC have the potential to regulate membrane function.

Effect of n-alkanols on lipid bilayer hydration

The effect of a homologous series of aliphatic n-alkanols on the presence of water within the head group and acyl chain region of lipid bilayers was investigated using time-resolved fluorescence spectroscopy according to a previously published approach [Ho, C., Slater, S. J., & Stubbs, C. D. (1995) Biochemistry 34, 6188-6195]. Upon addition of n-alkanols to phosphatidylcholine bilayers the fluorescence lifetime of N-[5-(dimethylamino)naphthalene-1-sulfonyl]dipalmitoylphosphatidyleth anolamine (dansyl-PE) decreased, indicative of an increased water content within the head group region, the effect being a linear function of n-alkanol chain length (C1-C8), based on the total n-alkanol concentration. The fluorescence lifetimes of 1-palmitoyl-2-[[2-[4-(6-phenyl-trans-1,3, 5-hexatrienyl)phenyl]ethyl]carbonyl]-3-sn-phosphatidylcholine (DPH-PC) and N-[[4-(6-phenyl-1,3, 5-hexatrienyl)phenyl]propyl]trimethylammonium p-toluenesulfonate (TMAP-DPH), and the fluorescence intensity ratio of the latter in D2O compared to that in H2O, were used to probe the level of water in the acyl chain region. There was a decrease in the lifetime and an increase in the D2O/H2O fluorescence intensity ratio upon addition of short-chain n-alkanols (C1-C3), suggestive of increased water content. By contrast, long-chain n-alkanols (C4-C8) increased the lifetime and decreased the ratio, suggestive of decreased water content. Acyl chain order, determined from DPH-PC fluorescence anisotropy, was decreased by all n-alkanols, indicating that the effects were not probe-dependent. The effects of short- and long-chain n-alkanols on the fluorescence lifetime of the tryptophans of gramicidin, incorporated into phosphatidylcholine bilayers as a model membrane protein, were similar to those obtained with TMAP-DPH and DPH-PC; ethanol decreased and hexanol increased the lifetime. Thus the effect of n-alkanols and general anesthetics on changes in the amount of water that may be accommodated within the acyl chain region of the bilayer is not predictable on the basis of the magnitude of effects on head group region or acyl chain order/fluidity.

Study of plasma membrane heterogeneity using a phosphatidylcholine derivative of 1,6-diphenyl-1,3,5-hexatriene [2-(3-(diphenylhexatriene)propanoyl)-3-palmitoyl-L-α-phosphatidylcholine]

The fluorescent probe 1,6-diphenyl-1,3,5-hexatriene (DPH) and DPH derivatives have been used to characterize structural and physicochemical properties of specific membrane domains. Steady-state and fluorescence decay measurements of three probes, DPH (1,6-diphenyl-1,3,5-hexatriene), TMA-DPH [1-(4-trimethyl-ammonium-phenyl)-6-phenyl-1,3,5-hexatriene], and a phosphatidylcholine derivative of DPH, DPH-pPC [2-(3-(diphenylhexatriene)propanoyl)-3-pamitoyl-L-α-phosphatidyl choline], have been performed in erythrocyte membranes and in lymphocyte plasma membranes. The steady-state fluorescence polarization of the three probes showed a similar trend in both membranes. In fact either in erythrocyte or in lymphocyte plasma membranes the fluorescence polarization values of DPH-pPC and TMA-DPH were similar, but significantly higher with respect to DPH. A better characterization of erythrocyte and lymphocyte plasma membranes was possible by using fluorescence decay measurements. The data suggest the possible use of different DPH derivatives to characterize specific domains in biological membranes.