Identification and characterization of polyphosphoinositides in the tonoplast membrane

In vitro data indicate that the tonoplast membrane is the major target site for IP3 action and Ca+2 is released from this compartment. If this is true, then several questions arise: (1) Is the plasma membrane or tonoplast the source of IP3? (2) If the plasma membrane is the source of IP3, how does IP3 get to the vacuole? In this connection, the results from in vivo labeling of carrot cells with [3H] inositol show that the major phospholipid components in the tonoplast are PI and lysoPIP. In vitro phosphorylation studies using [γ-32P]ATP indicate that PI kinase is present in the tonoplast membrane even though PIP was not detected with inositol labeling. If PIP and PIP2 are not present in the tonoplast, then IP3 must diffuse or be transported from the plasma membrane. Since IP3 is rapidly metabolized by cytosolic phosphatases there may be an IP3-sensitive calcium store other than the vacuole closely associated with the plasma membrane