Small membrane-associated GTP-binding proteins of catecholamine-secreting cells

1. Four GTP-binding proteins (23-27 kDa) were identified in membranes from PC12 cells by [alpha 32P]GTP binding to nitrocellulose blots of SDS-polyacrylamide gels. 2. The GTP-binding proteins remained associated with membranes during stimulation of intact cells by K(+)-depolarization or even after addition of Ca2+ to digitonin-permeabilized cells. 3. By two-dimensional gel electrophoresis, six GTP-binding proteins were resolved and based on their mobility, their phosphorylation state appeared independent of Ca2+. 4. Fractionation of PC12 membranes showed that these GTP-binding proteins were broadly distributed in post-nuclear membranes with the plasma membranes containing the highest specific GTP-binding activity. 5. Membrane fractions from bovine adrenal medulla contain similar GTP-binding proteins with GTP-binding intensity also being highest in the plasma membrane. 6. The GTP-binding proteins could be concentrated in the detergent-rich fraction upon Triton X-114 phase separation.