Inositol 1,4,5-trisphosphate-sensitive Ca2+ release across nonvacuolar membranes in cauliflower

Previous studies have indicated that the vacuole represents the major inositol 1,4,5-trisphosphate (InsP(3))-mobilizable Ca2+ pool in higher plants. This finding is in contrast to animal cells, in which the endoplasmic reticulum and plasma membrane constitute the dominant InsP(3)-sensitive membranes. We used membrane vesicles prepared from cauliflower (Brassica oleracae L.) inflorescences that were separated on continuous sucrose gradients to demonstrate that cauliflower possesses at least two distinct membrane populations that are sensitive to InsP(3). One of these membrane populations is nonvacuolar in origin and relies upon a Ca2+-ATPase to accumulate Ca2+. In addition, we have shown that two polyclonal antibodies, raised against peptides corresponding to the animal type 1 InsP(3) receptor, recognize immunologically related proteins in cauliflower, and that the distribution of immunoreactive proteins on a linear sucrose gradient reinforces the notion that cauliflower contains more than one membrane subtype that is sensitive to InsP(3). To our knowledge, this is the first report describing an InsP(3)-sensitive Ca2+ store other than the vacuole in higher plant cells.