Isolation and Characterization of Plasma Membranes from Wild Type

A method has been developed to isolate plasma membranes with high ATPase activity from wild type Neurospora. Cells are treated with snail enzyme to weaken their cell walls, disrupted by gentle homogenization in a medium designed to keep mitochondria and other organelles intact, and fractionated by differential centrifugation. After removal of mitochondria, several higher speed particulate fractions (particularly one sedimenting at 40,000 X g) contain an ATPase that can be identified as the plasma membrane enzyme on the basis of sensitivity to vanadate and kinetic properties. Its [S],E, for MgATP, specificity for nucleotides and divalent cations, and pH optimum are virtually identical with those reported previously for plasma membrane ATPase from the slime mutant of Neurospora (Bowman, B. J., and Slayman, C. W. (1977) J. Bwl. Chem 252, 3357-3363). By contrast, ATPase specific activities in the wild type plasma membranes are much higher than in dime, ranging up to 7.3 pmol/min/mg of protein (the highest value yet reported for Neurospora). The best preparations appear homogeneous upon sucrose density gradient centrifugation, and band at an equilibrium density of 1.15 g/cm3. Two other markers, chitin synthetase and [ ~ c e t y l ~ H ] concanavalin A binding, show approximate co-purification with the plasma membrane ATPase through membrane fractionation and sucrose gradient centrifugation.