Insight into the Structure of Lung Surfactant Protein B (SP-B)

Abstract

Lung surfactant proteins play crucial role in breathing and also help to defend the lungs from pathogens. Surfactant associated-protein B (SP-B) is the most vital component of lung surfactant. SP-B is essential to life in that it helps reduce the surface tension at the air-water interface of the alveolus to near zero value and consequently prevent the lung from collapse. However, SP-B's overall three-dimensional structure is not yet known, and therefore its mechanism of action remains a mystery. The extreme hydrophobic nature of SP-B makes it difficult to study. Nonetheless, recent advances in our laboratory have made it possible to recombinantly express surfactant protein B (rSP-B) in bacteria. Circular dichroism (CD) confirmed that rSP-B can be reproducibly and stably folded and can maintain its secondary structure over a range of temperatures. Dynamic Light scattering (DLS) indicates two size distributions by intensity, one at the right size range to study by solution nuclear magnetic resonance (NMR) and the other at a bigger size range. rSP-B is also showing promising result in vitro in terms of its functionality when tested with a Langmuir-Blodgett trough. Lipid films spread on water and compressed in the presence of an appropriate amount rSP-B are able to induce monolayer to multilayer transitions at the expected surface pressure of ∼40 to 50 mN/m and also show superior compressibility and faster adsorption rates compared to films compressed without rSP-B. Preliminary one-dimensional proton NMR spectra show signals in the amide peptide backbone region that are similar to those observed in earlier studies of SP-B fragments. Our findings indicate that recombinant SP-B can be stably folded and functional in vitro, and that it may be possible to find conditions to make it possible to determine SP-B's structure using solution NMR spectroscopy.