Vectorial labeling of dinoflagellate cell surface proteins

Our objective was to identify cell surface proteins in the dinoflagellate Lingulodinium polyedrum. Proteins on the surface of living cells that had regions exposed to the external medium were labeled with (NaI)-I-125. After partial purification of membrane proteins and analysis by two-dimensional gel electrophoresis and autoradiography, a protein of roughly 43 kDa was found to have incorporated the radiolabel. This protein was cloned using a combination of protein microsequencing and PCR amplification. The derived protein sequence in the cDNA has a signal peptide at the N-terminal end of the protein and thus represents the first plasma membrane protein ever reported for a dinoflagellate. The function of the protein is unknown, but its cloning provides a proof of principle for the general use of vectorial labeling in identifying cell surface proteins of marine algae.