Cell surface expression of DBA binding sites in LLC‐PK1 cells increases at confluence and is enhanced by PKA activation

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Abstract
Post‐confluent populations of LLC‐PK1 cells express many glycoproteins at the surface, including glycoproteins which bind the lectin Dolichos biflorus agglutinin (DBA). DBA‐binding glycoproteins are localized preferentially to the apical cell surface and exhibit molecular weights ranging from less than 30 kDa to greater than 200 kDa. Subconfluent cell populations exhibit little surface DBA binding. Upon attaining confluence DBA binding capacity increases progressively over several days. This correlates roughly with increasing activities of several differentiated apical membrane functions. Expression of DBA binding sites at the surface occurs on a cell‐by‐cell basis. An increasing level of surface DBA binding in the cell population corresponds to an increasing proportion of the cell population expressing binding sites. Activation of cAMP‐dependent protein kinase in post‐confluent cell populations, and to a lesser extent in subconfluent populations, increases surface DBA binding sites by a protein synthesis‐dependent mechanism after a lag period of at least 24 h. This results from increases in both the proportion of cells expressing surface binding sites and the level of binding sites on individual cells.
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