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

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.

Structural Features Essential to the Antimicrobial Functions of Human SPLUNC1

“…Second, it has been hypothesized that a surface-exposed leucine-rich loop in equine Latherin would be critical to surfactant activity. 28 In human SPLUNC1, that region corresponds to α4 (see Figs. 1C–D).…”

“…An elegant model for the surfactant actions of latherin has been proposed wherein this Leu-rich loop seeds the unrolling of the protein’s super-roll fold to expose its Leu-rich hydrophobic core at the air-water interface. 28,29 Here, we test this hypothesis for human SPLUNC1, and in doing so also address the structural basis of its ability to act as an LPS-binding and innate antibacterial factor. Together, the data presented support the conclusion that improved SPLUNC1s could be designed to act as highly effective protective factors of potential use in the diseased lungs of patients with CF or other pulmonary disorders.…”

Conformations of Myoglobin-Derived Peptides at the Air–Water Interface

“…For a small number of proteins, however, adsorption at interfaces is a key part of their function and these have evolved a number of strategies for interfacial adsorption. These include the amphiphilic surface structure of hydrophobins 2 or specific conformational changes seen in proteins such as ranaspumin, 3 latherin, 4 or lipase. 5 Non-surfactant proteins are also found to adsorb onto liquid interfaces and these are commonly used as foam or emulsion stabilisers, 6 with milk and whey proteins being used in food and other consumer products.…”

Palate Lung Nasal Clone (PLUNC), a Novel Protein of the Tear Film: Three-Dimensional Structure, Immune Activation, and Involvement in Dry Eye Disease (DED)

“…This model showed a very good overall quality and was generated based on the x-ray structure of latherin, a PLUNC family member with similar predicted fold. 28 This final PLUNC model was used for all further modeling experiments presented in this work.…”