Proteins of microdissected polytenic cells

Proteins extracted from unfixed and ethanol-acetic acid fixed salivary glands were compared by SDS-polyacrylamide gel electrophoresis. The electropherograms were nearly identical when care was taken to prevent degradation in the unfixed glands. Steady state proteins from microdissected nuclei and cytoplasms showed approximately the same number of species but displayed fundamentally different electrophoretic distributions. It was calculated that the maximum number of copies for individual polypeptide species ranged from 3.8×10⁸ to 1.8×10¹⁰ in large polytene nuclei. A comparison of electropherograms from steady state nuclear proteins and nuclear proteins labeled with ³H-leucine after various periods of in vitro incubation of the glands, may suggest different turnover rates for individual protein species. An unexpected effect of incubation of the explanted salivary glands in a synthetic medium was observed. There are differences, on a quantitative level, between certain labeled proteins which accumulate in the nucleus at the beginning and at the end of a relatively long period of incubation.