IMMUNOCHARACTERIZATION AND DEVELOPMENTAL REGULATION OF RABBIT LUNG CALCIUM-DEPENDENT PHOSPHOLIPID-BINDING PROTEINS

The purpose of this work was to use the immunoblotting methods to stud y the 36 kDa calcium-dependent phospholipid-binding protein (PLBP) in the adult and fetal rabbit lungs to gain insight into the significance of this protein in lung development. The identity of the 36 kDa PLBP and the antigen specificity of the antiserum raised against this prote in in the guinea pig were tested against known annexins and antibodies to the annexins. Our results showed that the rabbit lung 36 kDa PLBP contained only one protein which cross-reacted with antibodies against annexin 1. However, the 36 kDa PLBP was slightly smaller (36 vs. 37 k Da) and more acidic (pi 6.0 vs. 6.9) than the recombinant human annexi n 1. The guinea pig antiserum only reacted with annexin 1, not with an y of the other annexins tested. In the cytosolic fractions of the lung and the alveolar epithelial type II cells, and in the lung lavage flu id, the 36 kDa PLBP was by far the most prominent protein with minor p resence of a 33 kDa protein recognized by the guinea pig antiserum. Th e amount of the 36 kDa PLBP of type II cells was 55% higher than that in the lung tissue and 2.6-times higher than that in the lung lavage ( 9.3 +/- 0.62, 6.0 +/- 0.31 and 3.6 +/- 0.04 mu g/mg protein, respectiv ely). The 36 kDa PLBP appeared in the fetal rabbit lungs as early as a t 21 days gestation and increased 2-fold to reach the adult level at 2 7 days gestation (term 31 days). The high content of PLBP in type II c ells and the rapid increase in this protein in the fetal lungs at late gestations suggest an important role of the 36 kDa PLBP in lung devel opment and surfactant biogenesis.