Critical structural and functional roles for the N-terminal insertion sequence in surfactant protein B analogs.
BACKGROUND
Surfactant protein B (SP-B; 79 residues) belongs to the saposin protein superfamily, and plays functional roles in lung surfactant. The disulfide cross-linked, N- and C-terminal domains of SP-B have been theoretically predicted to fold as charged, amphipathic helices, suggesting their participation in surfactant activities. Earlier structural studies with Mini-B, a disulfide-linked construct based on the N- and C-terminal regions of SP-B (i.e., approximately residues 8-25 and 63-78), confirmed that these neighboring domains are helical; moreover, Mini-B retains critical in vitro and in vivo surfactant functions of the native protein. Here, we perform similar analyses on a Super Mini-B construct that has native SP-B residues (1-7) attached to the N-terminus of Mini-B, to test whether the N-terminal sequence is also involved in surfactant activity.


RESULTS
FTIR spectra of Mini-B and Super Mini-B in either lipids or lipid-mimics indicated that these peptides share similar conformations, with primary alpha-helix and secondary beta-sheet and loop-turns. Gel electrophoresis demonstrated that Super Mini-B was dimeric in SDS detergent-polyacrylamide, while Mini-B was monomeric. Surface plasmon resonance (SPR), predictive aggregation algorithms, and molecular dynamics (MD) and docking simulations further suggested a preliminary model for dimeric Super Mini-B, in which monomers self-associate to form a dimer peptide with a "saposin-like" fold. Similar to native SP-B, both Mini-B and Super Mini-B exhibit in vitro activity with spread films showing near-zero minimum surface tension during cycling using captive bubble surfactometry. In vivo, Super Mini-B demonstrates oxygenation and dynamic compliance that are greater than Mini-B and compare favorably to full-length SP-B.


CONCLUSIONS
Super Mini-B shows enhanced surfactant activity, probably due to the self-assembly of monomer peptide into dimer Super Mini-B that mimics the functions and putative structure of native SP-B.
Walther
Frans J
FJ
Los Angeles Biomedical Research Institute at Harbor, University of California Los Angeles Medical Center, Torrance, California, USA. fwalther@lumc.nl
Waring
Alan J
AJ
Hernandez-Juviel
Jose M
JM
Gordon
Larry M
LM
Wang
Zhengdong
Z
Jung
Chun-Ling
CL
Ruchala
Piotr
P
Clark
Andrew P
AP
Smith
Wesley M
WM
Sharma
Shantanu
S
Notter
Robert H
RH
D000595:Amino Acid Sequence; D000818:Animals; D004591:Electrophoresis, Polyacrylamide Gel; D008297:Male; D008958:Models, Molecular; D008969:Molecular Sequence Data; D008970:Molecular Weight; D011487:Protein Conformation; D037701:Pulmonary Surfactant-Associated Protein B / Q000737:chemistry / Q000502:physiology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D017550:Spectroscopy, Fourier Transform Infrared; D020349:Surface Plasmon Resonance
D037701:Pulmonary Surfactant-Associated Protein B