2 POLYPEPTIDES THAT ARE SECRETED FROM SUSPENSION-CULTURED TOBACCO CELLS IN THE PRESENCE OF BREFELDIN-A HAVE CHITIN-BINDING DOMAINS

Polypeptides SN9 and SN11 are secreted from suspension-cultured tobacco cells in the presence of Brefeldin A. Both polypeptides have N-terminal chitin-binding domains very similar to those of class I chitinases from tobacco. After SDS-polyacrylamide gel electrophoresis under denaturing conditions, the SN11 band exhibited chitinase activity. Antibodies directed against barley chitinase as well as against class I chitinases of tobacco cross-react with SN11. However, no cross-reaction took place with polypeptide SN9, which could also not degrade glycolchitin. Both polypeptides could be localized in vacsuoles of tobacco mesophyll protoplasts. Whereas SN11 seems to be one of the well characterized class I chitinases (EC 3.2.1.14) of tobacco, SN9 appears to be a lectin-like protein without chitinase activity. Experiments with SN11- and SN9-specific DNA and immunoprobes suggest that the Brefeldin A insensitivity of the secretion of SN9 and SN11 is due to a specific intracellular protein transport mechanism and not to Brefeldin A-induced synthesis of these polypeptides.