Organ specificity of a vacuolar Ca2+-binding protein RVCaB in radish and its expression under Ca2+-deficient conditions

Radish vacuoles contain a new type of Ca2+-binding protein (RVCaB) with hig h capacity and low affinity for Ca2+. The protein is able to stimulate Ca2 uptake into vacuoles, which is driven by Ca2+-ATPase and Ca2+/H+ antiporte r. In the present study, we found that the level of RVCaB mRNA is high in s eedling hypocotyls and mature taproots but low in young roots and mature le aves. The RVCaB protein was abundant in hypocotyls and taproots but absent in leaves. The levels of the transcript and protein of RVCaB in taproots we re gradually increased during maturation. The level of RVCaB mRNA in seedli ng hypocotyls doubled within a few hours when the growth medium was changed from 10 mM CaCl2 to water, although the level was strongly suppressed in 1 00 mM CaCl2. This response of the RVCaB gene was specific to Ca2+ and did n ot occur with other ions including K+ and Mg2+. RVCaB functioning as a Ca2-sequestering protein in taproot vacuoles to provide for the Ca2+ deficienc y is discussed.