A 42-KILODALTON ANNEXIN-LIKE PROTEIN IS ASSOCIATED WITH PLANT VACUOLES

A 42-kD, calcium-dependent, membrane-binding protein (VCaB42) was asso ciated with partially purified vacuole membranes. Membrane-dissociatio n assays indicated that VCaB42 binding to vacuole membranes was select ive for calcium over other cations and that 50% of VCaB42 remained mem brane bound at 61 +/- 11 nM free calcium. A 13-amino acid sequence obt ained from VCaB42 showed 85% similarity with the endonexin fold, a seq uence found in the annexin family of proteins that is thought to be es sential for calcium and lipid binding. The greatest similarity in amin o acid sequence was observed with annexin VIII (VAC-beta). The calcium -binding properties and sequence similarities suggest that VCaB42 is a member of the annexin family of calcium-dependent, membrane-binding p roteins. Functional assays for VCaB42 on vacuole membrane transport pr ocesses indicated that it did not significantly affect the initial rat e of calcium uptake into vacuole membrane vesicles. Because VCaB42 is vacuole localized (likely on the cytosolic surface of the vacuole) and is 50% dissociated within the physiological range of cytosolic free c alcium, we hypothesize that this protein is a sensor that monitors cyt osolic calcium levels and transmits that information to the vacuole.