Identification of pneumocin, a developmentally regulated apical membrane glycoprotein in rat lung type II and Clara cells.

Pneumocin (Mr, 165 kD) is a recently identified apical membrane surface sialoglycoprotein marker of type II pneumocytes. A murine monoclonal IgG1 subclass-producing clone 4A (4A mAb), which was developed against the purified pneumocin, and recognized pneumocin on Western blots of adult rat lung homogenates, was used to study expression of the glycoprotein in developing rat lungs. Pneumocin localized to apical membranes of late fetal, neonatal, and adult rat type II pneumocytes as well as Clara cells in situ, by immunofluorescence and immunoelectron microscopy. Faint immunofluorescence was observed in 17-d fetal lungs. However, 19-d fetal lungs showed intense immunofluorescence with the antibody. On immunoelectron microscopy, apical membranes of 19-d fetal and adult rat lung type II cells were labeled by 4A mAb, but type I cells were not stained. On Western blots, amounts of pneumocin increased up to the fourth day after birth, when near-adult levels were attained. Lower molecular weight forms (Mr, 80 to 90 kD) were recognized in 17-d fetal lung. These bands decreased in amount with a corresponding increase in the 165-kD band that was typically observed in adult lungs. Immunoglobulins that were eluted from polyvinylidene difluoride strips containing the 165-kD band recognized the Mr 80 to 90 kD bands and 50-kD component, suggesting that fetal forms of the protein shared an epitope in common with the adult pneumocin. Reactivity of the glycoprotein with 4A mAb was destroyed by enzymatic digestion with trypsin and staphylococcal V8 protease. These data demonstrate that pneumocin is a developmentally regulated apical membrane marker of differentiated type II and Clara cells.(ABSTRACT TRUNCATED AT 250 WORDS)