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The structure of latherin, a surfactant allergen protein from horse sweat and saliva.

Vance SJ, McDonald RE, Cooper A, Smith BO, Kennedy MW - J R Soc Interface (2013)

Bottom Line: Its surfactant activity is intrinsic to the protein in its native form, and is manifest without associated lipids or glycosylation.Intrinsically surface-active proteins are relatively rare in nature, and this is the first structure of such a protein from mammals to be reported.Both its conformation and proposed method of action are different from other, non-mammalian surfactant proteins investigated so far.

View Article: PubMed Central - PubMed

Affiliation: School of Chemistry, University of Glasgow, Glasgow G12 8QQ, UK.

ABSTRACT
Latherin is a highly surface-active allergen protein found in the sweat and saliva of horses and other equids. Its surfactant activity is intrinsic to the protein in its native form, and is manifest without associated lipids or glycosylation. Latherin probably functions as a wetting agent in evaporative cooling in horses, but it may also assist in mastication of fibrous food as well as inhibition of microbial biofilms. It is a member of the PLUNC family of proteins abundant in the oral cavity and saliva of mammals, one of which has also been shown to be a surfactant and capable of disrupting microbial biofilms. How these proteins work as surfactants while remaining soluble and cell membrane-compatible is not known. Nor have their structures previously been reported. We have used protein nuclear magnetic resonance spectroscopy to determine the conformation and dynamics of latherin in aqueous solution. The protein is a monomer in solution with a slightly curved cylindrical structure exhibiting a 'super-roll' motif comprising a four-stranded anti-parallel β-sheet and two opposing α-helices which twist along the long axis of the cylinder. One end of the molecule has prominent, flexible loops that contain a number of apolar amino acid side chains. This, together with previous biophysical observations, leads us to a plausible mechanism for surfactant activity in which the molecule is first localized to the non-polar interface via these loops, and then unfolds and flattens to expose its hydrophobic interior to the air or non-polar surface. Intrinsically surface-active proteins are relatively rare in nature, and this is the first structure of such a protein from mammals to be reported. Both its conformation and proposed method of action are different from other, non-mammalian surfactant proteins investigated so far.

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Distribution of leucine and isoleucine residues in the latherin structure. Latherin's main chain is displayed in cartoon representation, with the solvent accessible surface envelope shown in transparency. Leucine side chains are displayed as yellow spheres, and isoleucine side chains as orange spheres. In (a), the ‘loop’ end is at the top and the ‘termini’ end at the bottom. (b) and (c) show views from the ‘loop’ and ‘termini’ ends, respectively. Leucine and isoleucine residues predominantly line the core of the fold except at the ‘loop’ end. Image created using PyMOL [46].
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RSIF20130453F3: Distribution of leucine and isoleucine residues in the latherin structure. Latherin's main chain is displayed in cartoon representation, with the solvent accessible surface envelope shown in transparency. Leucine side chains are displayed as yellow spheres, and isoleucine side chains as orange spheres. In (a), the ‘loop’ end is at the top and the ‘termini’ end at the bottom. (b) and (c) show views from the ‘loop’ and ‘termini’ ends, respectively. Leucine and isoleucine residues predominantly line the core of the fold except at the ‘loop’ end. Image created using PyMOL [46].

Mentions: The structure of latherin displays little evidence of any amphiphilicity that might have been anticipated by comparison with the distinct patches of polar and apolar side chains seen on the surface of hydrophobins [59–61]. By contrast, the exterior of latherin in the bulk phase, shows no such surface patches, being almost exclusively decorated with the side chains of hydrophilic residues and predominantly anionic due to the higher proportion of aspartic and glutamic acids over arginines, histidines and lysines (figure 1c,d). This is intriguing and initially unexpected, especially in view of the unusually high proportion of leucines in latherin (49 of the 208 residues), a trait common to human SPLUNC1, which also exhibits surfactant activity [12]. In the latherin structure, the leucines are evenly distributed along the length of the structure, being mainly confined to the interior in the ordered regions of the protein (figure 3a). But, at the loop end, about one-third of all the leucines are exposed to solvent (cf. figure 3b for loop end, and figure 3c for termini end; and see the electronic supplementary material, table S3 for numerical comparison). That these loop leucines and other adjacent aliphatic residues do not form an obvious hydrophobic patch is in part because their polar main-chain groups are solvent exposed and also because they are interspersed with polar residues.Figure 3.


The structure of latherin, a surfactant allergen protein from horse sweat and saliva.

Vance SJ, McDonald RE, Cooper A, Smith BO, Kennedy MW - J R Soc Interface (2013)

Distribution of leucine and isoleucine residues in the latherin structure. Latherin's main chain is displayed in cartoon representation, with the solvent accessible surface envelope shown in transparency. Leucine side chains are displayed as yellow spheres, and isoleucine side chains as orange spheres. In (a), the ‘loop’ end is at the top and the ‘termini’ end at the bottom. (b) and (c) show views from the ‘loop’ and ‘termini’ ends, respectively. Leucine and isoleucine residues predominantly line the core of the fold except at the ‘loop’ end. Image created using PyMOL [46].
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4043175&req=5

RSIF20130453F3: Distribution of leucine and isoleucine residues in the latherin structure. Latherin's main chain is displayed in cartoon representation, with the solvent accessible surface envelope shown in transparency. Leucine side chains are displayed as yellow spheres, and isoleucine side chains as orange spheres. In (a), the ‘loop’ end is at the top and the ‘termini’ end at the bottom. (b) and (c) show views from the ‘loop’ and ‘termini’ ends, respectively. Leucine and isoleucine residues predominantly line the core of the fold except at the ‘loop’ end. Image created using PyMOL [46].
Mentions: The structure of latherin displays little evidence of any amphiphilicity that might have been anticipated by comparison with the distinct patches of polar and apolar side chains seen on the surface of hydrophobins [59–61]. By contrast, the exterior of latherin in the bulk phase, shows no such surface patches, being almost exclusively decorated with the side chains of hydrophilic residues and predominantly anionic due to the higher proportion of aspartic and glutamic acids over arginines, histidines and lysines (figure 1c,d). This is intriguing and initially unexpected, especially in view of the unusually high proportion of leucines in latherin (49 of the 208 residues), a trait common to human SPLUNC1, which also exhibits surfactant activity [12]. In the latherin structure, the leucines are evenly distributed along the length of the structure, being mainly confined to the interior in the ordered regions of the protein (figure 3a). But, at the loop end, about one-third of all the leucines are exposed to solvent (cf. figure 3b for loop end, and figure 3c for termini end; and see the electronic supplementary material, table S3 for numerical comparison). That these loop leucines and other adjacent aliphatic residues do not form an obvious hydrophobic patch is in part because their polar main-chain groups are solvent exposed and also because they are interspersed with polar residues.Figure 3.

Bottom Line: Its surfactant activity is intrinsic to the protein in its native form, and is manifest without associated lipids or glycosylation.Intrinsically surface-active proteins are relatively rare in nature, and this is the first structure of such a protein from mammals to be reported.Both its conformation and proposed method of action are different from other, non-mammalian surfactant proteins investigated so far.

View Article: PubMed Central - PubMed

Affiliation: School of Chemistry, University of Glasgow, Glasgow G12 8QQ, UK.

ABSTRACT
Latherin is a highly surface-active allergen protein found in the sweat and saliva of horses and other equids. Its surfactant activity is intrinsic to the protein in its native form, and is manifest without associated lipids or glycosylation. Latherin probably functions as a wetting agent in evaporative cooling in horses, but it may also assist in mastication of fibrous food as well as inhibition of microbial biofilms. It is a member of the PLUNC family of proteins abundant in the oral cavity and saliva of mammals, one of which has also been shown to be a surfactant and capable of disrupting microbial biofilms. How these proteins work as surfactants while remaining soluble and cell membrane-compatible is not known. Nor have their structures previously been reported. We have used protein nuclear magnetic resonance spectroscopy to determine the conformation and dynamics of latherin in aqueous solution. The protein is a monomer in solution with a slightly curved cylindrical structure exhibiting a 'super-roll' motif comprising a four-stranded anti-parallel β-sheet and two opposing α-helices which twist along the long axis of the cylinder. One end of the molecule has prominent, flexible loops that contain a number of apolar amino acid side chains. This, together with previous biophysical observations, leads us to a plausible mechanism for surfactant activity in which the molecule is first localized to the non-polar interface via these loops, and then unfolds and flattens to expose its hydrophobic interior to the air or non-polar surface. Intrinsically surface-active proteins are relatively rare in nature, and this is the first structure of such a protein from mammals to be reported. Both its conformation and proposed method of action are different from other, non-mammalian surfactant proteins investigated so far.

Show MeSH
Related in: MedlinePlus