CHARACTERIZATION AND IMMUNOHISTOCHEMICAL LOCALIZATION OF THE 15 KD PROTEIN ISOLATED FROM RAT LUNG LAMELLAR BODIES

We have characterized a protein of approximately 15 kD (lb15) derived from rat lung lamellar bodies, and then sequenced the first 42 residues. Following the normal isopycnic sucrose gradient ultracentrifugation, we diluted the band containing the crude lamellar body fraction with an equal volume of cold distilled water and further centrifuged it at 2,000 x g for 30 min to pellet a fraction of lamellar bodies. Under the electron microscope, this fraction appeared intact and highly purified. When this fraction was subjected to polyacrylamide gel electrophoresis, the major protein was one of 15 kD, regardless of whether the fraction was extracted or unextracted, reduced or unreduced; only a small amount of 35 kD protein was detected with Coomassie Blue staining. Disruption of lamellar bodies revealed that the limiting membrane was particularly enriched with lb15. Immunohistochemistry indicated that lb15 was present in lamellar bodies and tubular myelin, suggesting it was secreted along with the lipid. Amino acid analysis revealed a protein with 13.5% basic and 10.6% acidic residues. The N-terminal appeared particularly highly charged, with 32% of the charged residues in the first 14 amino acids. The lb15 protein is identical to rat lysozyme for the first 23 residues, with the important exception of residue 6, which is histidine in lb15 and cysteine in lysozyme. Residue 24 was not identified. Lb15 was also present in lavage material. We conclude that lb15 is the major protein in rat lung lamellar bodies, has a highly charged N-terminal, and shares some sequence homology with rat lysozyme. We suggest that this protein is possibly a variation of lysozyme and is involved in the packaging of lipid.