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

The microsomal fraction from mung-bean (Vigna radiata) hypocotyl was f ound to contain Ins(1,4,5)P-3- and Ins(2,4,5)P-3-binding activity. Pre incubation of the microsomal fraction with thiol-containing reagents r educed 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 pro tein. The binding protein was purified to homogeneity and the molecula r mass of the native form recorded as 400 kDa. However, under denaturi ng conditions the molecular mass was 110 kDa, suggesting that the prot ein is a homotetramer. That this protein is associated with Ca2+ relea se 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 highe r 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 Ins P(3) with characteristics of high affinity and low density.