Role of surfactant protein A (SP-A)/lipid interactions for SP-A functions in the lung

Surfactant protein A (SP-A), an oligomeric glycoprotein, is a member of a g roup of proteins named collectins that contain collagen-like and Ca2+-depen dent carbohydrate recognition domains. SP-A interacts with a broad range of amphipathic lipids (glycerophospholipids, sphingophospholipids, glycosphin golipids, lipid A, and lipoglycans) that are present in surfactant or micro bial membranes. This review summarizes SP-A/lipid interaction studies regar ding the lipid system used (i.e., phospholipid vesicles, phospholipid monol ayers, and lipids immobilized on silica or adsorbed on a solid support). Th e effect of calcium, ionic strength, and pH on the binding of SP-A to lipid s and the subsequent lipid aggregation process is discussed. Current eviden ce suggests that hydrophobic-binding forces are involved in the peripherica l association of SP-A to membranes. It is also proposed that fluid and liqu id-ordered phase coexistence in surfactant membranes might favor partition of SP-A into those membranes. The binding of SP-A to surfactant membranes c ontaining hydrophobic surfactant peptides makes possible the formation of a membrane reservoir in the alveolar fluid that is protected by SP-A against inactivation and improves the rate of surfactant film formation. In additi on, the interaction of SP-A with membranes might enhance the affinity of SP -A for terminal carbohydrates of glycolipids or glycoproteins on the surfac e of invading microorganisms.