Structure and orientation of the surfactant‐associated protein C in a lipid bilayer

Abstract

The secondary structure of native and depalmitoylated porcine surfactant-associated protein C (SP-C) was studied by attenuated total reflection Fourier-transform infrared spectroscopy. Both forms of porcine SP-C adopt mainly an alpha-helical conformation. These two forms of the protein were reconstituted in a lipid bilayer. The insertion of the protein in a membrane is associated with an increase of the alpha-helical content. Dichroic measurements show that, in both cases, the long axis of the alpha-helix is oriented parallel to the lipid acyl chains.

The 21‐residue surfactant peptide (LysLeu4)4Lys(KL4) is a transmembrane α‐helix with a mixed nonpolar/polar surface

“…The FTIR spectrum of KL4 in DPPC/PG 7:3 (w/w) bilayers shows a peak maximum at 1656 cm -1, in the spectral region associated with helical structure. Deconvolution and curve fitting between 1700 cm -1 and 1600 cm -1 [8,12,23] yields 84% of a-helices for lipid-associated KL4. KL4 is helical also in non-lipid environments; in trifluoroethanol it gives a CD spectrum which is similar to that obtained in DPC micelles, and FTIR spectroscopy of KL4 alone, i.e.…”

“…interface Spreading kinetics of 2% (w/w) of KL4 in a DPPC/PG/PA 68:22:9 (by weight) mixture at an air/water interface was compared to that of native SP-C and SP-C/BR [20], both with transmembranous c~-helices in similar phospholipid mixtures [11,12,14,26]. KL4 is less efficient than native SP-C and SP-C/BR in accelerating the spreading of the lipid mixture and also reaches a lower surface pressure after 1 min (Fig.…”

“…SP-C is a 35-residue polypeptide [10] with palmitoyl groups thioester-linked to the cysteine residues in the N-terminal part of the molecule [6]. FTIR spectroscopy revealed that SP-C adopts mainly an ahelical conformation with its a-helix oriented parallel to the lipid acyl chains [11,12]. These restults are supported by those obtained by NMR spectroscopy showing that SP-C in mixed organic solvents forms a very regular a-helix between positions 9 and 34 [13] which is perfectly suited to span a DPPC bilayer [14].…”

Molecular Structures and Interactions of Pulmonary Surfactant Components

“…Alternatively, it could be brought about via global destabilization of the helix with concomitant conversion of a fraction of peptides into a non-helical conformation. The latter explanation appears less likely since non-helical SP-C has a pronounced tendency to aggregate and Vandenbussche et al (1992b) analyzed the secondary structure of depalmitoylated SP-C in phospholipid bilayers after isolation of the peptidellipid vesicles by sucrose density gradient centrifugation, whereby unbound peptides and vesicles containing non-helical/aggregated peptides are expected to be removed. In the NMR structure determination (Johansson et al, 1994b) no NOEs between protons of the palmitoyl groups and the peptide were found.…”

“…The procedure used for isolation may affect the structure of at least SP-C, which has been shown to exhibit a tendency to denature and aggregate under various solvent conditions (Ptrez-Gil et al, 1993 ;Johansson et al, 1994bJohansson et al, , 1995a. Furthermore, chemically depalmitoylated SP-C (see below) exhibits a different secondary structure compared to the native lipopeptide (Vandenbussche et al, 1992b;Johansson et al, 1995a) and gives a significantly lower yield on Sephadex LH-60 chromatography (Curstedt et al, unpublished data). Thus, data from studies on SP-C where different isolation procedures have been used may not be directly comparable.…”