Identification of canine pulmonary surfactant-associated glycoprotein A precursors

Surfactant-associated glycoproteins A were identified by two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis of crude surfactant from canine alveolar lavage: an unglycosylated form (protein A1), 27,000-28,000 daltons; glycoprotein A2, 32,000-34,000 daltons; and glycoprotein A3, 37,000-38,000 daltons; pH at isoelectric point (pI) 4.5-5.0. Glycoproteins A2 and A3 were electroeluted and used to prepare a monospecific antiserum that identified proteins A1, A2, and A3 in immunoblots of crude surfactant obtained from dog lung lavage. This antiserum precipitated several proteins from in vitro translated canine lung poly(A)+ mRNA; proteins of 27,000 daltons, pI 5.0, and 28,000 daltons, pI 4.8-5.0, which precisely comigrated with proteins A1 from canine surfactant. Cotranslational processing of the primary translation products by canine pancreatic microsomal membranes resulted in larger proteins of 31,000-34,000 daltons, pI 4.8-5.0. Treatment of these processed forms of glycoprotein A with endoglycosidase F, to remove N-linked carbohydrate, resulted in proteins of 27,000-28,000 daltons which precisely comigrated with surfactant protein A1. These observations demonstrate that the polypeptide precursors to the glycoproteins A complex are extensively modified by addition of asparagine N-linked complex carbohydrate and are subsequently secreted as glycoproteins A2 and A3.