IDENTIFICATION AND CHARACTERIZATION OF HIGH-MOLECULAR-MASS MUCIN-LIKEGLYCOPROTEINS IN THE PLASMA-MEMBRANE OF AIRWAY EPITHELIAL-CELLS

A previous lectin binding study demonstrated the presence of high mole cular-mass mucin-like glycoproteins (HMGP) on the surface of hamster t racheal surface epithelial (HTSE) secretory cells (Proc. Natl. Acad. S ci. USA 1987;84.9304). In the present study, we intended to isolate an d characterize these HMGP from the plasma membrane of the primary HTSE cells and then to determine whether or not these membrane HMGP are Mu c-1 mucins, a type of mucins originally discovered on the surface of s ome carcinomas. A subcellular fraction enriched with the plasma membra ne was obtained using a sucrose density gradient centrifugation. This fraction contained high molecular-mass glycoconjugates which were excl uded from Sepharose CL-4B gel. Biochemical characterization of these g lycoconjugates revealed the following characteristics: (1) susceptibil ity to both pronase and mild alkaline treatments, but totally resistan t to proteoglycan-digesting enzymes; (2) partitioning in the detergent phase of Triton X-114 and resistance to digestion by phosphatidylinos itol phospholipase C or D; (3) a buoyant density of 1.5 g/ml based on CsCl density gradient centrifugation; (4) polydispersity in terms of b oth size and charge density; and (5) lack of immunoreactivity with an anti-Muc-1 mucin antibody. We conclude that the plasma membrane of HTS E cells at confluence contains HMGP, which seem to be the integral mem brane proteins but different from Muc-1 mucins, and that these membran e HMGP appear to share some similarities with secreted mucins in terms of size and charge.