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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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Topology of latherin and comparison with BPI. (a) Topology model of latherin. α-Helices are represented by red rectangles; β-strands by yellow arrows; non-regular secondary structure as green lines. The intramolecular disulfide bond, Cys133–Cys175, is shown as a cyan line labelled ‘S–S’. The short section of π-helix is coloured orange, and the β-bulge by a curved green line between strands 1′ and 1″. (b) Cartoon representation of latherin compared with, (c) and (d), the N- and C-terminal domains, respectively, of BPI protein (PDB code 1BP1; [47]). The grey mesh in (d) encloses the internal cavities in the BPI C-terminal domain that are accessible to a 1.4 Å radius probe. Images created using PyMOL [46].
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RSIF20130453F2: Topology of latherin and comparison with BPI. (a) Topology model of latherin. α-Helices are represented by red rectangles; β-strands by yellow arrows; non-regular secondary structure as green lines. The intramolecular disulfide bond, Cys133–Cys175, is shown as a cyan line labelled ‘S–S’. The short section of π-helix is coloured orange, and the β-bulge by a curved green line between strands 1′ and 1″. (b) Cartoon representation of latherin compared with, (c) and (d), the N- and C-terminal domains, respectively, of BPI protein (PDB code 1BP1; [47]). The grey mesh in (d) encloses the internal cavities in the BPI C-terminal domain that are accessible to a 1.4 Å radius probe. Images created using PyMOL [46].

Mentions: The N-terminal helix, labelled αA, stretches from residues 7–47 with two breaks in the regular secondary structure at residues 22–23 and 31–33. Helix αA can therefore be subdivided into three sections: (7–21), and (see topology diagram in figure 2a). Helix αB (152–203) is also interrupted such that and are separated by a short region of π-helix as indicated by the signature i to i + 5 hydrogen bonding between the amides of residues 171–172 and the carbonyl groups of 166–167, respectively. Sections of π-helix can destabilize a helix and are often associated with functional sites [48–51]. Helix αC comprises residues 188–203. The four strands that make up the anti-parallel β-sheet are β1 (61–77), β2 (83–97), β3 (104–120) and β4 (126–142). β1 is interrupted by a β-bulge at a point where a proline (Pro89) in strand 2 introduces an irregularity in the packing of the two strands and does not present a hydrogen bonding partner to strand β1. It can therefore be divided into two sections and . The disulfide bond (Cys133–Cys176) connects β4 to .Figure 2.


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)

Topology of latherin and comparison with BPI. (a) Topology model of latherin. α-Helices are represented by red rectangles; β-strands by yellow arrows; non-regular secondary structure as green lines. The intramolecular disulfide bond, Cys133–Cys175, is shown as a cyan line labelled ‘S–S’. The short section of π-helix is coloured orange, and the β-bulge by a curved green line between strands 1′ and 1″. (b) Cartoon representation of latherin compared with, (c) and (d), the N- and C-terminal domains, respectively, of BPI protein (PDB code 1BP1; [47]). The grey mesh in (d) encloses the internal cavities in the BPI C-terminal domain that are accessible to a 1.4 Å radius probe. Images created using PyMOL [46].
© Copyright Policy - open-access
Related In: Results  -  Collection

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

RSIF20130453F2: Topology of latherin and comparison with BPI. (a) Topology model of latherin. α-Helices are represented by red rectangles; β-strands by yellow arrows; non-regular secondary structure as green lines. The intramolecular disulfide bond, Cys133–Cys175, is shown as a cyan line labelled ‘S–S’. The short section of π-helix is coloured orange, and the β-bulge by a curved green line between strands 1′ and 1″. (b) Cartoon representation of latherin compared with, (c) and (d), the N- and C-terminal domains, respectively, of BPI protein (PDB code 1BP1; [47]). The grey mesh in (d) encloses the internal cavities in the BPI C-terminal domain that are accessible to a 1.4 Å radius probe. Images created using PyMOL [46].
Mentions: The N-terminal helix, labelled αA, stretches from residues 7–47 with two breaks in the regular secondary structure at residues 22–23 and 31–33. Helix αA can therefore be subdivided into three sections: (7–21), and (see topology diagram in figure 2a). Helix αB (152–203) is also interrupted such that and are separated by a short region of π-helix as indicated by the signature i to i + 5 hydrogen bonding between the amides of residues 171–172 and the carbonyl groups of 166–167, respectively. Sections of π-helix can destabilize a helix and are often associated with functional sites [48–51]. Helix αC comprises residues 188–203. The four strands that make up the anti-parallel β-sheet are β1 (61–77), β2 (83–97), β3 (104–120) and β4 (126–142). β1 is interrupted by a β-bulge at a point where a proline (Pro89) in strand 2 introduces an irregularity in the packing of the two strands and does not present a hydrogen bonding partner to strand β1. It can therefore be divided into two sections and . The disulfide bond (Cys133–Cys176) connects β4 to .Figure 2.

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