Cell Surface Localization of the Sialyltransferase Ectoenzyme System During the Chlamydomonas Mating Reaction

Abstract

Glycosyltransferase-acceptor activity was demonstrated previously with gametes of Chlamydomonas moewusii and was shown to be enhanced during the mating reaction (McLean and Bosmann, 1975). This investigation is intended to provide some supportive data for the surface localization of sialyltransferase activity and to determine the possibility of hydrolysis of the CMP-sialic acid substrate by surface hydrolases resulting in uptake of the labelled sialic acid. The data indicate that Chlamydomonas cannot utilize sialic acid £or growth in the dark although it is taken up by the cell. Free sialic acid uptake is not enhanced during mating. An excess of free sialic acid did not suppress sialyltransferase activity thus weakening the possibility that CMP-sialic acid, the donor in the reaction, was hydrolyzed resulting in uptake of the free monosaccharide. Trypsinization of gametes before mating significantly reduced the level of activity in the transferase assay. Trypsinization after mating removed nearly all of the label incorporated during the assay. These data support the observation that sialyltransferase-acceptor activity detected on Chlamydomonas gametes is surface-localized and associated with the mating reaction.