CELLULAR SITE OF SYNTHESIS AND DYNAMICS OF CELL-SURFACE REEXPRESSION OF POLYSIALIC ACID OF THE NEURAL CELL-ADHESION MOLECULE

Homopolymers of alpha-2,8-ketosidically linked sialic acid (polysialic acid) represent a posttranslational modification which, in mammals, a ppears to be unique for the neural cell adhesion molecule and the cr s ubunit of sodium channels in brain. Under steady-state conditions, pol ysialic acid is detectable in the plasma membrane of different cell ty pes but not in the cytoplasm. We have studied the site of synthesis an d the cell surface re-expression of polysialic acid in a clonal sublin e of small cell lung carcinoma using the monoclonal antibody 735 and b acteriophage endosialidase, both specific reagents for polysialic acid . After enzymic removal, cell surface polysialic acid re-expression re ached 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-synt hesized polysialic acid became detectable in the Golgi apparatus. Our data show that synthesis of polysialic acid of the neural cell adhesio n molecule with a degree of polymerization of at least nine occurs int racellular in the Golgi apparatus of a human small cell lung carcinoma cell line.