MEMBRANE-BOUND FORMS OF CA2+-DEPENDENT PROTEIN MODULATOR - CA2+-DEPENDENT AND INDEPENDENT BINDING OF MODULATOR PROTEIN TO PARTICULATE FRACTION FROM BRAIN

  • 1 January 1978
    • journal article
    • research article
    • Vol. 4  (3) , 219-231
Abstract
Ca2+-dependent binding of modulator protein to the particulate fraction was studied. The particulate fraction from 1 g of rat brain bound in a Ca2+-dependent fashion 144 .mu.g of modulator protein, representing > 1/3 of the total soluble modulator protein in this tissue. The binding site was present in both the mitochondrial and microsomal fractions, the specific activity of the microsomes being the higher. The binding was reversible with a physiological concentration of Ca2+, and was temperature-dependent, and the site can be saturated with modulator protein (4.5 .mu.g modulator protein/mg of microsomal protein). Tryptic digestion of the membranes caused complete disappearance of the binding activity, but heat-treatment for 5 min at 70.degree. C caused only 40% loss of activity. The binding site may be a known or unknown enzyme(s) the activity of which is regulated by Ca2+ and modulator. Alternatively, this binding site may be a nonenzymic protein that regulates the concentration of free modulator protein in the cell. A Ca2+-independent type of binding of modulator protein to the membranes was also found. This type of membrane-bound modulator was not released from the membranes by treating the membranes with EGTA [ethylene glycol bis (.beta.-aminoethylether) tetraacetic acid] solution or trypsin.