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Lung surfactant protein A (SP-A) interactions with model lung surfactant lipids and an SP-B fragment.

Sarker M, Jackman D, Booth V - Biochemistry (2011)

Bottom Line: We have also probed SP-A's interaction with Mini-B, a biologically active synthetic fragment of SP-B, in the presence of micelles.Despite variations in Mini-B's own interactions with micelles of different compositions, SP-A is found to interact with Mini-B in all micelle systems and perhaps to undergo a further structural rearrangement upon interacting with Mini-B.The degree of SP-A-Mini-B interaction appears to be dependent on the type of lipid headgroup and is likely mediated through the micelles, rather than direct binding.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics and Physical Oceanography, Memorial University of Newfoundland, St. John's, NL, Canada.

ABSTRACT
Surfactant protein A (SP-A) is the most abundant protein component of lung surfactant, a complex mixture of proteins and lipids. SP-A performs host defense activities and modulates the biophysical properties of surfactant in concerted action with surfactant protein B (SP-B). Current models of lung surfactant mechanism generally assume SP-A functions in its octadecameric form. However, one of the findings of this study is that when SP-A is bound to detergent and lipid micelles that mimic lung surfactant phospholipids, it exists predominantly as smaller oligomers, in sharp contrast to the much larger forms observed when alone in water. These investigations were carried out in sodium dodecyl sulfate (SDS), dodecylphosphocholine (DPC), lysomyristoylphosphatidylcholine (LMPC), lysomyristoylphosphatidylglycerol (LMPG), and mixed LMPC + LMPG micelles, using solution and diffusion nuclear magnetic resonance (NMR) spectroscopy. We have also probed SP-A's interaction with Mini-B, a biologically active synthetic fragment of SP-B, in the presence of micelles. Despite variations in Mini-B's own interactions with micelles of different compositions, SP-A is found to interact with Mini-B in all micelle systems and perhaps to undergo a further structural rearrangement upon interacting with Mini-B. The degree of SP-A-Mini-B interaction appears to be dependent on the type of lipid headgroup and is likely mediated through the micelles, rather than direct binding.

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Translational diffusion measurements of SP-A in water, SDS, and DPC micelles. Top panels show the 2D DOSY spectra of 0.2 mM SP-A in water (A), 0.2 mM SP-A in 40 mM SDS (B), and 0.2 mM SP-A in 40 mM DPC (C). Bottom panels show the linear fits obtained for the attenuation of the integrated HN region of SP-A in water (D), in complex with SDS (E), and in complex with DPC (F). The linear fits for pure SDS (40 mM) and DPC (40 mM) micelles, obtained from the attenuation of the peak at 0.80 ppm, are included in (E) and (F) for comparison.
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fig2: Translational diffusion measurements of SP-A in water, SDS, and DPC micelles. Top panels show the 2D DOSY spectra of 0.2 mM SP-A in water (A), 0.2 mM SP-A in 40 mM SDS (B), and 0.2 mM SP-A in 40 mM DPC (C). Bottom panels show the linear fits obtained for the attenuation of the integrated HN region of SP-A in water (D), in complex with SDS (E), and in complex with DPC (F). The linear fits for pure SDS (40 mM) and DPC (40 mM) micelles, obtained from the attenuation of the peak at 0.80 ppm, are included in (E) and (F) for comparison.

Mentions: To address this apparently substantial change of the SP-A oligomerization state upon addition of micelles, 2D DOSY experiments were performed to estimate the size of the complexes (Figures 2 and 3, Table 1). For SP-A alone in water, the apparent hydrodynamic diameter, dHA, is 11.11 ± 1.48 nm. By contrast, the dHA of SP-A–micelle complexes are much smaller in SDS and DPC. For SP-A–SDS, the dHA are 3.27 ± 0.93 and 6.30 ± 0.94 nm, as measured using the SDS peaks and SP-A peaks, respectively. Similarly, for SP-A–DPC, the dHA measured from the DPC peaks and SP-A peaks are 3.57 ± 0.21 and 4.20 ± 0.21 nm, respectively. The dHA of pure micelles were also measured for comparison and found to be 1.22 ± 0.02 nm for SDS and 1.96 ± 0.01 for DPC, which conform well to what has been found by others for low SDS concentrations.(46) It is normal to obtain different diffusion coefficients from the detergent or lipid peaks compared to the protein peaks of a protein–micelle sample, since the observed value is the weighted average of the free and bound species.(47) And, this allows for the calculation of the relative populations of free and protein-bound micelles. On the basis of the application of a two-site model (Supporting Information, Table S1),(45) it is found that 76% of the SDS micelles and 85% of the DPC micelles are bound to SP-A, while the rest remain as protein-free micelles.


Lung surfactant protein A (SP-A) interactions with model lung surfactant lipids and an SP-B fragment.

Sarker M, Jackman D, Booth V - Biochemistry (2011)

Translational diffusion measurements of SP-A in water, SDS, and DPC micelles. Top panels show the 2D DOSY spectra of 0.2 mM SP-A in water (A), 0.2 mM SP-A in 40 mM SDS (B), and 0.2 mM SP-A in 40 mM DPC (C). Bottom panels show the linear fits obtained for the attenuation of the integrated HN region of SP-A in water (D), in complex with SDS (E), and in complex with DPC (F). The linear fits for pure SDS (40 mM) and DPC (40 mM) micelles, obtained from the attenuation of the peak at 0.80 ppm, are included in (E) and (F) for comparison.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3104520&req=5

fig2: Translational diffusion measurements of SP-A in water, SDS, and DPC micelles. Top panels show the 2D DOSY spectra of 0.2 mM SP-A in water (A), 0.2 mM SP-A in 40 mM SDS (B), and 0.2 mM SP-A in 40 mM DPC (C). Bottom panels show the linear fits obtained for the attenuation of the integrated HN region of SP-A in water (D), in complex with SDS (E), and in complex with DPC (F). The linear fits for pure SDS (40 mM) and DPC (40 mM) micelles, obtained from the attenuation of the peak at 0.80 ppm, are included in (E) and (F) for comparison.
Mentions: To address this apparently substantial change of the SP-A oligomerization state upon addition of micelles, 2D DOSY experiments were performed to estimate the size of the complexes (Figures 2 and 3, Table 1). For SP-A alone in water, the apparent hydrodynamic diameter, dHA, is 11.11 ± 1.48 nm. By contrast, the dHA of SP-A–micelle complexes are much smaller in SDS and DPC. For SP-A–SDS, the dHA are 3.27 ± 0.93 and 6.30 ± 0.94 nm, as measured using the SDS peaks and SP-A peaks, respectively. Similarly, for SP-A–DPC, the dHA measured from the DPC peaks and SP-A peaks are 3.57 ± 0.21 and 4.20 ± 0.21 nm, respectively. The dHA of pure micelles were also measured for comparison and found to be 1.22 ± 0.02 nm for SDS and 1.96 ± 0.01 for DPC, which conform well to what has been found by others for low SDS concentrations.(46) It is normal to obtain different diffusion coefficients from the detergent or lipid peaks compared to the protein peaks of a protein–micelle sample, since the observed value is the weighted average of the free and bound species.(47) And, this allows for the calculation of the relative populations of free and protein-bound micelles. On the basis of the application of a two-site model (Supporting Information, Table S1),(45) it is found that 76% of the SDS micelles and 85% of the DPC micelles are bound to SP-A, while the rest remain as protein-free micelles.

Bottom Line: We have also probed SP-A's interaction with Mini-B, a biologically active synthetic fragment of SP-B, in the presence of micelles.Despite variations in Mini-B's own interactions with micelles of different compositions, SP-A is found to interact with Mini-B in all micelle systems and perhaps to undergo a further structural rearrangement upon interacting with Mini-B.The degree of SP-A-Mini-B interaction appears to be dependent on the type of lipid headgroup and is likely mediated through the micelles, rather than direct binding.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics and Physical Oceanography, Memorial University of Newfoundland, St. John's, NL, Canada.

ABSTRACT
Surfactant protein A (SP-A) is the most abundant protein component of lung surfactant, a complex mixture of proteins and lipids. SP-A performs host defense activities and modulates the biophysical properties of surfactant in concerted action with surfactant protein B (SP-B). Current models of lung surfactant mechanism generally assume SP-A functions in its octadecameric form. However, one of the findings of this study is that when SP-A is bound to detergent and lipid micelles that mimic lung surfactant phospholipids, it exists predominantly as smaller oligomers, in sharp contrast to the much larger forms observed when alone in water. These investigations were carried out in sodium dodecyl sulfate (SDS), dodecylphosphocholine (DPC), lysomyristoylphosphatidylcholine (LMPC), lysomyristoylphosphatidylglycerol (LMPG), and mixed LMPC + LMPG micelles, using solution and diffusion nuclear magnetic resonance (NMR) spectroscopy. We have also probed SP-A's interaction with Mini-B, a biologically active synthetic fragment of SP-B, in the presence of micelles. Despite variations in Mini-B's own interactions with micelles of different compositions, SP-A is found to interact with Mini-B in all micelle systems and perhaps to undergo a further structural rearrangement upon interacting with Mini-B. The degree of SP-A-Mini-B interaction appears to be dependent on the type of lipid headgroup and is likely mediated through the micelles, rather than direct binding.

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