Intracellular migration of nuclear proteins in Xenopus oocytes

Abstract

THE protein composition of the amphibian oocyte nucleus (germinal vesicle, or GV) is very different from that of the cytoplasm, as shown by SDS electrophoresis^1–3. The oocyte nucleus is known to accumulate components such as RNA polymerase⁴, histones⁵ and factors that protect DNA from nucleases⁶, probably stockpiled for use during early development. Furthermore, some nuclear proteins accumulate in the germinal vesicle after microinjection into the oocyte cytoplasm, as shown by Gurdon⁷ for labelled histones and by Bonner⁸, who injected labelled total GV contents into oocytes. We describe here a study of these phenomena using a recently described high resolution method of protein analysis involving two-dimensional gel electrophoresis⁹. Having defined those proteins which are nuclear, cytoplasmic, or present in both cell compartments of Xenopus laevis oocytes, we micro-injected a soluble preparation of labelled GV proteins into oocyte cytoplasm and analysed their distribution in the nuclear and cytoplasmic compartments. The results show that cells have exquisitely selective mechanisms that determine the extent to which a protein will become concentrated in the nucleus. Our findings suggest that nuclear proteins contain in their molecular structure a signal that enables them to accumulate in the nucleus. This signal is absent in proteins such as actin, which are similarly abundant in both nucleus and cytoplasm.