RECEPTOR FOR MYOINOSITOL TRISPHOSPHATE FROM THE MICROSOMAL FRACTION OF VIGNA-RADIATA

The microsomal fraction from mung-bean (Vigna radiata) hypocotyl was found to contain Ins(1,4,5)P-3- and Ins(2,4,5)P-3-binding activity. Preincubation of the microsomal fraction with thiol-containing reagents reduced specific InsP(3) binding. A single class of binding site with a K-d value of 1.5 nM and B-max, of 1.1 pmol/mg of protein was detected. Other myo-inositol phosphates exhibited little affinity for this protein. The binding protein was purified to homogeneity and the molecular mass of the native form recorded as 400 kDa. However, under denaturing conditions the molecular mass was 110 kDa, suggesting that the protein is a homotetramer. That this protein is associated with Ca2+ release was confirmed by including it in proteoliposomes and adding Ins(1,4,5)P-3 or Ins(2,4,5)P-3. The affinity if Ins(1,4,5)P-3 is 3-fold higher than that of Ins(2,4,5)P-3. The binding affinity of InsP(3) is also reflected in the extent of Ca2+ released from the microsomal fraction. Heparin inhibits binding of InsP(3) to the protein, the K-1/2 being 0.26 mu M. It is also shown that the protein acts as a receptor for InsP(3) with characteristics of high affinity and low density.