Cotton fiber annexins: A potential role in the regulation of callose synthase

Cotton fibers contain a characteristic set of proteins which interact with plasma membranes in a Ca²⁺-dependent manner. The association of these proteins with the membrane is correlated with a reduced level of UDP-glucose: (1→3)-β-glucan (callose) synthase activity. Analysis of the proteins released from membranes by EDTA treatment shows that the most abundant proteins comprise a family of at least three polypeptides (p34) which resemble annexins. This resemblance includes similarity in size (about 34 kDa), sequence homology, Ca²⁺-dependent precipitation or interaction with the plasma membrane, and ability to serve as a substrate for phosphorylation by endogenous protein kinase(s) which also bind to the membranes in a Ca²⁺-dependent manner. A purified fraction of these annexins binds to, and inhibits, the activity of a partially purified cotton fiber callose synthase. These findings suggest that one possible function of annexin(s) in plants is to modulate the activity and/or localization of callose synthase.