A VACUOLE-ASSOCIATED ANNEXIN PROTEIN, VCAB42, CORRELATES WITH THE EXPANSION OF TOBACCO CELLS

A Ca-dependent membrane-binding protein of the annexin family, VCaB42, has previously been shown to associate with vacuolar vesicles at phys iological levels of Ca. In this study we used suspension-cultured cell s of tobacco (Nicotiana tabacum BY-2) to show that VCaB42 is enriched 4.5-fold in intact vacuoles, whereas evacuolated protoplasts show a 12 -fold reduction in VCaB42. VCaB42 distribution is thus comparable to t hat of the vacuole-associated H+-ATPase but is distinct from the endop lasmic reticulum-localized protein calnexin. Because VCaB42 is a vacuo le-associated annexin, and given the putative function of annexins in vesicle fusion, we hypothesize a role for this protein in the vacuolat ion process of expanding cells. Consistent with this hypothesis, we sh ow that VCaB42 levels correlate with age-associated and hormonally ind uced changes in cell volume in tobacco suspension cultures. The associ ation of VCaB42 with vacuoles and its correlative pattern of expressio n relative to the expansion of cells is consistent with a possible rol e for VCaB42 in the early events of vacuole biogenesis.