Abstract
Homopolymers of alpha-2,8-ketosidically linked sialic acid (polysialic acid) represent a posttranslational modification which, in mammals, appears to be unique for the neural cell adhesion molecule and the alpha subunit of sodium channels in brain. Under steady-state conditions, polysialic acid is detectable in the plasma membrane of different cell types but not in the cytoplasm. We have studied the site of synthesis and the cell surface re-expression of polysialic acid in a clonal subline of small cell lung carcinoma using the monoclonal antibody 735 and bacteriophage endosialidase, both specific reagents for polysialic acid. After enzymic removal, cell surface polysialic acid re-expression reached control levels only after 5 days. When Golgi to plasma membrane transport of endosialidase-treated cells was blocked by culture at 20 degrees C or in the presence of monensin at 37 degrees C, de-novo-synthesized polysialic acid became detectable in the Golgi apparatus. Our data show that synthesis of polysialic acid of the neural cell adhesion molecule with a degree of polymerization of at least nine occurs intracellular in the Golgi apparatus of a human small cell lung carcinoma cell line.
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