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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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Signal attenuation curves obtained from the translational diffusion measurements of 0.1 mM SP-A + 0.1 mM Mini-B in 40 mM SDS (A) and in 40 mM DPC (B). None of the curves fit well with a single line. However, approximately the first and the last halves of the data fit well with two lines having two different slopes. Consequently, two diffusion coefficients are obtained for each system.
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fig5: Signal attenuation curves obtained from the translational diffusion measurements of 0.1 mM SP-A + 0.1 mM Mini-B in 40 mM SDS (A) and in 40 mM DPC (B). None of the curves fit well with a single line. However, approximately the first and the last halves of the data fit well with two lines having two different slopes. Consequently, two diffusion coefficients are obtained for each system.

Mentions: Since 2D HSQC spectra suggested that, upon addition of SP-A, there was likely a substantial increase in the size of Mini-B complexes in zwitterionic micelles but no major change in anionic or mixed micelles, we performed translational diffusion measurements to probe the change in size for all systems. Interestingly, 2D DOSY spectra of the SP-A–Mini-B mixture, when compared to that of the individual proteins, demonstrate a change in dHA for all micelle compositions. As shown in Figure 5, the signal attenuation curves for SP-A–Mini-B mixtures in SDS and DPC micelles do not fit well with a single line (i.e., a single component fit). However, approximately the first and the last halves of the data are fit well with two lines having two different slopes [i.e., a two-component fit(48)]. Thus, two diffusion coefficients are obtained, and there are, at least, two distinct subpopulations of protein–micelle complexes present in the sample. The diffusion coefficients and corresponding hydrodynamic diameters measured from the two fits are reported in Table 1. In SDS, the dHA of the SP-A–Mini-B subpopulations are 6.61 ± 0.27 and 20.02 ± 0.86 nm, as measured from the protein peaks. Although the dHA of the first subpopulation is not significantly different from the SP-A–SDS complex (6.30 ± 0.94 nm), that of the second subpopulation is much larger. Hence, a fraction of the total Mini-B and SP-A molecules present in the mixture likely form large combined protein–micelle complexes. The approximate ratio of the small-to-large subpopulations of Mini-B–SP-A–SDS is 85%:15%, as estimated from the y-axis (relative signal intensity) intercepts of the two linear fits for the HN signal attenuation. In DPC, on the other hand, the dHA of the SP-A–Mini-B subpopulations are 11.80 ± 1.48 and 20.07 ± 0.43 nm as measured from the protein peaks. In this case, the dHA of both subpopulations are much larger than that of SP-A–DPC (4.20 ± 0.21 nm) or Mini-B–DPC (2.56 ± 0.35 nm) complexes. The approximate ratio of small-to-large subpopulations of Mini-B–SP-A–DPC is 62%:38%, as estimated from the y-axis intercepts of the two linear fits for the HN signal attenuation. Thus, in DPC, perhaps the entire populations of SP-A and Mini-B interact to form larger complexes, but with heterogeneous sizes.


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)

Signal attenuation curves obtained from the translational diffusion measurements of 0.1 mM SP-A + 0.1 mM Mini-B in 40 mM SDS (A) and in 40 mM DPC (B). None of the curves fit well with a single line. However, approximately the first and the last halves of the data fit well with two lines having two different slopes. Consequently, two diffusion coefficients are obtained for each system.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

fig5: Signal attenuation curves obtained from the translational diffusion measurements of 0.1 mM SP-A + 0.1 mM Mini-B in 40 mM SDS (A) and in 40 mM DPC (B). None of the curves fit well with a single line. However, approximately the first and the last halves of the data fit well with two lines having two different slopes. Consequently, two diffusion coefficients are obtained for each system.
Mentions: Since 2D HSQC spectra suggested that, upon addition of SP-A, there was likely a substantial increase in the size of Mini-B complexes in zwitterionic micelles but no major change in anionic or mixed micelles, we performed translational diffusion measurements to probe the change in size for all systems. Interestingly, 2D DOSY spectra of the SP-A–Mini-B mixture, when compared to that of the individual proteins, demonstrate a change in dHA for all micelle compositions. As shown in Figure 5, the signal attenuation curves for SP-A–Mini-B mixtures in SDS and DPC micelles do not fit well with a single line (i.e., a single component fit). However, approximately the first and the last halves of the data are fit well with two lines having two different slopes [i.e., a two-component fit(48)]. Thus, two diffusion coefficients are obtained, and there are, at least, two distinct subpopulations of protein–micelle complexes present in the sample. The diffusion coefficients and corresponding hydrodynamic diameters measured from the two fits are reported in Table 1. In SDS, the dHA of the SP-A–Mini-B subpopulations are 6.61 ± 0.27 and 20.02 ± 0.86 nm, as measured from the protein peaks. Although the dHA of the first subpopulation is not significantly different from the SP-A–SDS complex (6.30 ± 0.94 nm), that of the second subpopulation is much larger. Hence, a fraction of the total Mini-B and SP-A molecules present in the mixture likely form large combined protein–micelle complexes. The approximate ratio of the small-to-large subpopulations of Mini-B–SP-A–SDS is 85%:15%, as estimated from the y-axis (relative signal intensity) intercepts of the two linear fits for the HN signal attenuation. In DPC, on the other hand, the dHA of the SP-A–Mini-B subpopulations are 11.80 ± 1.48 and 20.07 ± 0.43 nm as measured from the protein peaks. In this case, the dHA of both subpopulations are much larger than that of SP-A–DPC (4.20 ± 0.21 nm) or Mini-B–DPC (2.56 ± 0.35 nm) complexes. The approximate ratio of small-to-large subpopulations of Mini-B–SP-A–DPC is 62%:38%, as estimated from the y-axis intercepts of the two linear fits for the HN signal attenuation. Thus, in DPC, perhaps the entire populations of SP-A and Mini-B interact to form larger complexes, but with heterogeneous sizes.

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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