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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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Comparison of the average apparent hydrodynamic diameters (dHA) of SP-A in water, pure micelles, individual SP-A– and Mini-B–micelle complexes, and combined SP-A–Mini-B–micelle complexes as calculated from the 2D DOSY NMR spectra.
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fig6: Comparison of the average apparent hydrodynamic diameters (dHA) of SP-A in water, pure micelles, individual SP-A– and Mini-B–micelle complexes, and combined SP-A–Mini-B–micelle complexes as calculated from the 2D DOSY NMR spectra.

Mentions: Diffusion measurements demonstrate that dHA for micelles of all compositions increase substantially upon addition of SP-A, reflecting the formation of detergent/lipid–protein complexes (Table 1 and Figure 6). An analysis based on a two-site model (Supporting Information, Table S1)(45) indicates that, for SDS and DPC systems, more than three-fourths of the micelles are involved in the formation of complexes with SP-A. Interestingly, the dHA for SP-A–SDS and SP-A–DPC complexes are more than 3 times smaller than that for SP-A alone in the absence of micelles. However, for SP-A in complex with LMPC, LMPG, or LMPC+LMPG micelles, the dHA are similar to that of SP-A alone in water. To interpret what this means in terms of the oligomeric state of SP-A within various micelles, we have estimated the contribution of the micelle itself to the diffusion of the complex and used this to estimate the oligomeric state of the SP-A within the micelle–SP-A complex. This analysis indicates SP-A oligomeric states of approximately 10, 12, and 9 molecules in LMPC, LMPG, and LMPC+LMPG micelles, respectively, as well as oligomeric states of approximately 1 and 3 molecules in DPC and SDS micelles, respectively (Supporting Information, Table S5). In the absence of micelles, the diffusion measurements indicate an oligomeric state of even larger than octadecamer (18 molecules) for SP-A. Comparison of SP-A’s NMR signal intensity with that of Mini-B (Supporting Information, Table S3) indicates that the vast majority of SP-A molecules, if not the full population, are observed in the NMR spectra acquired in the presence of micelles; i.e., the observed signals are not generated by just a small subpopulation of SP-A. There is, therefore, a dramatic reduction in SP-A’s oligomeric state when the protein is bound to 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)

Comparison of the average apparent hydrodynamic diameters (dHA) of SP-A in water, pure micelles, individual SP-A– and Mini-B–micelle complexes, and combined SP-A–Mini-B–micelle complexes as calculated from the 2D DOSY NMR spectra.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

fig6: Comparison of the average apparent hydrodynamic diameters (dHA) of SP-A in water, pure micelles, individual SP-A– and Mini-B–micelle complexes, and combined SP-A–Mini-B–micelle complexes as calculated from the 2D DOSY NMR spectra.
Mentions: Diffusion measurements demonstrate that dHA for micelles of all compositions increase substantially upon addition of SP-A, reflecting the formation of detergent/lipid–protein complexes (Table 1 and Figure 6). An analysis based on a two-site model (Supporting Information, Table S1)(45) indicates that, for SDS and DPC systems, more than three-fourths of the micelles are involved in the formation of complexes with SP-A. Interestingly, the dHA for SP-A–SDS and SP-A–DPC complexes are more than 3 times smaller than that for SP-A alone in the absence of micelles. However, for SP-A in complex with LMPC, LMPG, or LMPC+LMPG micelles, the dHA are similar to that of SP-A alone in water. To interpret what this means in terms of the oligomeric state of SP-A within various micelles, we have estimated the contribution of the micelle itself to the diffusion of the complex and used this to estimate the oligomeric state of the SP-A within the micelle–SP-A complex. This analysis indicates SP-A oligomeric states of approximately 10, 12, and 9 molecules in LMPC, LMPG, and LMPC+LMPG micelles, respectively, as well as oligomeric states of approximately 1 and 3 molecules in DPC and SDS micelles, respectively (Supporting Information, Table S5). In the absence of micelles, the diffusion measurements indicate an oligomeric state of even larger than octadecamer (18 molecules) for SP-A. Comparison of SP-A’s NMR signal intensity with that of Mini-B (Supporting Information, Table S3) indicates that the vast majority of SP-A molecules, if not the full population, are observed in the NMR spectra acquired in the presence of micelles; i.e., the observed signals are not generated by just a small subpopulation of SP-A. There is, therefore, a dramatic reduction in SP-A’s oligomeric state when the protein is bound to 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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