The cDNA for the full-length lectin from the marine sponge Geodia cydonium was cloned. Analysis of the deduced aa sequence revealed that this lectin belongs to the group of galectins. The full-length galectin, which was obtained also in a recombinant form, has an M(r) of 20,877; in the processed form it is a 15 kDa polypeptide. The enriched aggregation factor from G.cydonium also was determined to contain, besides minimal amounts of the galectin, a 140 kDa polypeptide which is involved in cell-cell adhesion. Monoclonal antibodies have been raised against this protein; Fab' fragments prepared from them abolished cell-cell reaggregation. Cell reaggregation experiments revealed that the aggregation factor is an essential component in the aggregation process but it requires likewise the presence of the galectin. Antibodies against the galectin blocked the aggregation factor-mediated cell adhesion. A plasma membrane component was identified (a 29 kDa polypeptide) which interacted with the aggregation factor in the presence of galectin; binding could be blocked both by antibodies against the galectin as well as against the aggregation factor. Immunohistochemical analysis revealed that spherulous cells contain the galectin but not the aggregation factor. By laser scanning microscopy, it is shown that both the aggregation factor and the galectin are located at the rim of the cells. From these data we conclude that the G.cydonium aggregation factor binds to the cells via a galectin bridge.

ABSTRACT. The potential role of inorganic polyphosphates in the response of algal cells to osmotic stress was investigated. We show that changes in polyphosphate metabolism of algae induced by osn~otic stress can be easily determined by measuring the metachromatic shift of toluldine blue produced by polyphosphate and by analysing the size of the extracted polyphosphate on urea/polyacrylamide gels. The amount and size of polyphosphate were found to differ considerably among different algal species. It is demonstrated that the alga Phaeodactylum tricornutum responds to hyperosmotic stress with a marked elongation of polyphosphate molecules and a decrease in the total amount of extractable polyphosphate, while exposure to hypoosmotic stress results in an increase in soluble shorter-chain polyphosphates and a rise in total polyphosphate content.

No abstract

No abstract