Elucidation of Lipid Binding Sites on Lung Surfactant Protein A Using X‑ray Crystallography, Mutagenesis, and Molecular Dynamics Simulations

Surfactant protein A (SP-A) is a
collagenous C-type lectin (collectin)
that is critical for pulmonary defense against inhaled microorganisms.
Bifunctional avidity of SP-A for pathogen-associated molecular patterns
(PAMPs) such as lipid A and for dipalmitoylphosphatidylcholine (DPPC),
the major component of surfactant membranes lining the air–liquid
interface of the lung, ensures that the protein is poised for first-line
interactions with inhaled pathogens. To improve our understanding
of the motifs that are required for interactions with microbes and
surfactant structures, we explored the role of the tyrosine-rich binding
surface on the carbohydrate recognition domain of SP-A in the interaction
with DPPC and lipid A using crystallography, site-directed mutagenesis,
and molecular dynamics simulations. Critical binding features for
DPPC binding include a three-walled tyrosine cage that binds the choline
headgroup through cation−π interactions and a positively
charged cluster that binds the phosphoryl group. This basic cluster
is also critical for binding of lipid A, a bacterial PAMP and target
for SP-A. Molecular dynamics simulations further predict that SP-A
binds lipid A more tightly than DPPC. These results suggest that the
differential binding properties of SP-A favor transfer of the protein
from surfactant DPPC to pathogen membranes containing appropriate
lipid PAMPs to effect key host defense functions.