Purification and characterization of two rabbit lung Ca2(+)-dependent phospholipid-binding proteins

Two Ca2(+)-dependent phospholipid-binding proteins (PLBPs) in rabbit lung cytosolic fraction have been purified to homogeneity. The apparent molecular weights of these two proteins are 36,000 and 33,000. Both the 36,000 and 33,000 PLBPs aggregated certain negatively charged unilamellar liposomes, but not the neutral phosphatidylcholine (PC) liposomes, in the presence of Ca2+. However, both PLBPs fused PC unilamellar liposomes to membrane acceptors. The 36,000 and 33,000 PLBPs had different specificities for phospholipid head groups, effects of Ca2+ and membrane charges and amino acid compositions. Both the PLBPs aggregated the surfactant membranes (lamellar bodies or from lung lavage) and nonsurfactant membranes (microsomes or mitochondria) to a level similar to that of the synthetic acidic phospholipid vesicles, but the proteins fused [14C]PC liposomes to the surfactant membranes 13- to 16-times more than to the synthetic phospholipid vesicles. The 36,000 PLBP fused [14C]PC liposomes to microsomes or mitochondria only half that of the fusion to the surfactant; the 33,000 PLBP fused [14C]PC liposomes to the surfactant and nonsurfactant alike. The PLBP's aggregate activity was not affected by the depletion of biological membrane proteins and the disruption of the native membrane integrity, but its fusion activity was greatly reduced. These results suggest that: (1) the 36,000 and 33,000 PLBPs are two different proteins; (2) the PLBPs may possess two catalytic reactions, one for the aggregation of small vesicles due to the binding of the protein to phospholipid vesicles and one for the fusion of small vesicles to acceptors; (3) the fusion activity was probably regulated by biological membrane proteins or structures; and (4) lung PLBP(s) might play a role in lung surfactant biogenesis.