Surfactant protein B: lipid interactions of synthetic peptides representing the amino-terminal amphipathic domain.

Abstract

The mechanisms by which pulmonary surfactant protein B (SP-B) affects the surface activity of surfactant lipids are unclear. We have studied the peptide/lipid interactions of the amino-terminal amphipathic domain of SP-B by comparing the secondary conformations and surface activities of a family of synthetic peptides based on the native human SP-B sequence, modified by site-specific amino acid substitutions. Circular dichroism measurements show an ahelical structure correlating with the ability of the peptides to interact with lipids and with the surface activity of peptide/lipid dispersions. Amino acid substitutions altering either the charge or the hydrophobicity of the residues lowered the helical content and reduced the association of the aminoterminal segment with lipid dispersions. Surface activity of peptide/lipid mixtures was maximally altered by reversal of charge in synthetic peptides. These observations indicate that electrostatic interactions and hydrophobicity are important factors in determining optimal structure and function of surfactant peptides in lipid dispersions.Surfactant protein B (SP-B, Mr 9000; refs. 1 and 2) is encoded on chromosome 2 and synthesized by type 2 pneumocytes (3-6). The primary translation product for SP-B is a 381-residue glycosylated preprotein (Mr 42,000) (7). Amino-and carboxyl-terminal proteolytic cleavage produces a 79-residue peptide, with amino terminus located at residue 201 of the preprotein. Mature SP-B lacks glycosylation sites, is highly hydrophobic, and has a basic pl. In contrast to surfactant protein C (SP-C), SP-B has no covalently linked lipids, although it coisolates with SP-C and surfactant lipids (8). Four of 10 positively charged residues (two arginines, two lysines) appear at the amino terminus. Negatively and positively charged residues are found at the carboxyl terminus (one aspartate, one glutamate, four arginines). Intra-and intermolecular sulfhydryl bonds are important structural features of the mature protein, which contains seven cysteines and is oligomeric under oxidizing conditions (9, 10).Together with surfactant protein A (SP-A) and SP-C, SP-B interacts with lipids to form mammalian pulmonary surfactant. SP-B is present in lamellar bodies in type II pneumocytes and appears in bronchoalveolar lavage. SP-B from bovine and porcine sources contributes to the function of surfactant preparations used in replacement therapies (11)(12)(13)(14)(15). Synthetic SP-B (16) and synthetic peptides from predicted amphipathic sequences of human SP-B (17) emulate native SP-B in surfactant lipid dispersions (13,(16)(17)(18)(19).Quantitative measurements of the amphipathicity of the human amino acid sequence of SP-B using the Eisenberg hydrophobic algorithm (20) predict that the amino-terminal sequence has a high a hydrophobic moment (9, 21). Therefore, we investigated the amino-terminal sequence of SP-B to identify secondary structures and specific conformations associated with surface activity in peptide/lipid dispersions.Based on the native amino-te...

The relationships between biophysical activity and the secondary structure of synthetic peptides from the pulmonary surfactant protein SP‐B

“…Tierney Trough area (em~2) conformations when complexed with phospholipids or SDS or in TFE (23,24). To evaluate structure-surfactant activity relationships, we measured the CD spectra of the purified bovine SP-B and the peptides derived from the C-terminal regions of human and bovine SP-B in several environments (Fig.…”

The proteins of the surfactant system

“…Synthetic peptides corresponding to some parts of native SP-B, when associated with phospholipids, possess some biophysical and biological activity [239,[256][257][258][259][260], but are clearly inferior to native SP-B in both respects. Also, synthetic peptides with simplified sequences designed to mimic specific parts of the SP-B polypeptide [258,260], and amphipathic α-helical peptides based on SP-A [261], exhibit some surface activity in combination with phospholipids.…”

“…Also, synthetic peptides with simplified sequences designed to mimic specific parts of the SP-B polypeptide [258,260], and amphipathic α-helical peptides based on SP-A [261], exhibit some surface activity in combination with phospholipids. However, although encouraging, such results should perhaps be taken somewhat cautiously, since an amphipathic α-helical decapeptide with a sequence unrelated to SP-B [262], or simple amino acid homopolymers [263] combined with phospholipids also exhibit biophysical activities that are superior to phospholipids alone, but inferior to natural pulmonary surfactant.…”

Molecular Structures and Interactions of Pulmonary Surfactant Components

“…Several predictions of the locations of regular secondary structure elements in SP-B have been put forward (Takahashi et al, 1990;Bruni et al, 1991;Whitsett and Baatz, 1992;Keough, 1992;Waring et al, 1993;Cruz et al, 1995;Andersson et al, 1995 a). A common feature is that several amphipathic helices have been predicted but their number and locations in the amino acid sequence have varied.…”