Role of the N-Terminal Seven Residues of Surfactant Protein B (SP-B)

Abstract

Breathing is enabled by lung surfactant, a mixture of proteins and lipids that forms a surface-active layer and reduces surface tension at the air-water interface in lungs. Surfactant protein B (SP-B) is an essential component of lung surfactant. In this study we probe the mechanism underlying the important functional contributions made by the N-terminal 7 residues of SP-B, a region sometimes called the “insertion sequence”. These studies employed a construct of SP-B, SP-B (1–25,63–78), also called Super Mini-B, which is a 41-residue peptide with internal disulfide bonds comprising the N-terminal 7-residue insertion sequence and the N- and C-terminal helices of SP-B. Circular dichroism, solution NMR, and solid state 2H NMR were used to study the structure of SP-B (1–25,63–78) and its interactions with phospholipid bilayers. Comparison of results for SP-B (8–25,63–78) and SP-B (1–25,63–78) demonstrates that the presence of the 7-residue insertion sequence induces substantial disorder near the centre of the lipid bilayer, but without a major disruption of the overall mechanical orientation of the bilayers. This observation suggests the insertion sequence is unlikely to penetrate deeply into the bilayer. The 7-residue insertion sequence substantially increases the solution NMR linewidths, most likely due to an increase in global dynamics.

Quenching of tryptophan fluorescence in a highly scattering solution: Insights on protein localization in a lung surfactant formulation

“…The SP-B analogue of CHF5633 lacks the first 7 aminoacids with respect to full length, wt SP-B. The 7 residues at the N-terminus constitute the membrane insertion sequence, which is reported to cause a dramatic increase in the acyl chain disorder in the center of the bilayer [ 44 ]. The Trp residue of SP-B analogue of CHF5633 reports the same polarity as Trp of peptides presenting the 7 amino acids insertion sequence, suggesting an unaltered location and a maintained capacity of anchoring on the membrane surface.…”

All-Atom Molecular Dynamics Simulations of Dimeric Lung Surfactant Protein B in Lipid Multilayers

“…A variety of structural features of SP-B have previously been suggested to contribute to SP-B-mediated lipid structural transitions, and will be considered in turn in the light of this work’s simulations of SP-B dimers. Firstly, the N-terminal seven residues of SP-B, with sequence FPIPLPY, have been termed the “insertion sequence” and have been proposed to be important in helping SP-B insert into the air/lipid interface [34,35]. In all simulations, with the exception of the CO2 system, this region of SP-B was observed to localize tightly to the polar/apolar interface of the lipids.…”

“…It is even possible that SP-B’s topology changes during the respiratory cycle (i.e., as the air/water surface is compressed upon expiration and expands upon inspiration), or that the SP-B topology is different in different LS lipid structures. One additional structural feature to note is the seven N-terminal residues of SP-B, with sequence FPIPLPY, which have been termed the “insertion sequence” and have been proposed to help SP-B insert into the lipid monolayer at the air/water interface [34,35].…”

Synthetic surfactants with SP‐B and SP‐C analogues to enable worldwide treatment of neonatal respiratory distress syndrome and other lung diseases

“…Adding these seven residues to Mini-B, resulting in an analogue referred to as Super Mini-B increases the surfactant activity in vitro and in vivo [182,183]. The CD spectra of the N-terminal heptapeptide alone confirmed that it does not form an a-helical structure, and Super Mini-B shows similar contents of an a-helical structure such as Mini-B [181]. Super Mini-B also appears to be a more stable dimer at submicellar SDS concentrations than Mini-B, thereby mimicking better the native SP-B.…”

“…b) . Further studies have suggested an important role of the N‐terminal hydrophobic seven‐residue segment of SP‐B (FPIPLPY, Table ), that are not included in Mini‐B . The three Pro in this motif probably efficiently prevent the formation of an α‐helix and the high hydrophobicity could promote interactions with surfactant phospholipid membranes.…”