The Subcellular Localization of Cerebral Phosphoproteins Sensitive to Electrical Stimulation.

1. On incubating cerebral-cortex slices at 37 degrees in an oxygenated medium marked changes resulted in the subcellular distribution of proteins and phosphoproteins in the tissue. The protein content of the nuclear fraction more than doubled, whereas the yields of microsomal and supernatant proteins were both markedly decreased. The amount of phosphoprotein/mg. of protein decreased in the microsomal and supernatant fractions, but showed little change in the nuclear and mitochondrial fractions. The loss of microsomal protein could be partly prevented by rinsing the slices briefly in cold sucrose solution before dispersion; the altered subcellular distribution was apparently related to contamination of the dispersing solution with traces of salts from the medium. 2. The subcellular location of the phosphoprotein sensitive to the effects of electrical pulses applied to cerebral slices in vitro has been reinvestigated by two different procedures. Comparison between unstimulated and stimulated slices after incubation in the presence of [(32)P]orthophosphate showed that phosphoprotein radioactivity increased on stimulation to a greater extent in a membrane-rich fraction than in a mitochondria-rich fraction, these being obtained by immediate density-gradient fractionation of the tissue dispersion. With fractions isolated by differential centrifuging the percentage increase in a combined mitochondrial and nuclear fraction was 5% as compared with 24% (P<0.02) in the microsomal fraction and 30% in the original dispersion before fractionation. The sensitive phosphoprotein therefore appears to be located in structures sedimenting with the microsomal fraction, rather than with the nuclear fraction as previously claimed.