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 tobacc o cells in the presence of Brefeldin A. Both polypeptides have N-termi nal chitin-binding domains very similar to those of class I chitinases from tobacco. After SDS-polyacrylamide gel electrophoresis under dena turing conditions, the SN11 band exhibited chitinase activity. Antibod ies directed against barley chitinase as well as against class I chiti nases of tobacco cross-react with SN11. However, no cross-reaction too k place with polypeptide SN9, which could also not degrade glycolchiti n. Both polypeptides could be localized in vacsuoles of tobacco mesoph yll protoplasts. Whereas SN11 seems to be one of the well characterize d class I chitinases (EC 3.2.1.14) of tobacco, SN9 appears to be a lec tin-like protein without chitinase activity. Experiments with SN11- an d SN9-specific DNA and immunoprobes suggest that the Brefeldin A insen sitivity of the secretion of SN9 and SN11 is due to a specific intrace llular protein transport mechanism and not to Brefeldin A-induced synt hesis of these polypeptides.