A method to examine the diffusible state and the sizes of major cytoplasmic proteins in a living cell is described. Small (40-300 microns) commercially available gel filtration beads of a broad range of Mr exclusion limits were microsurgically implanted into the cytoplasm of oocytes of the frog, Xenopus laevis, usually after metabolic labeling of oocyte proteins with [35S]methionine. After equilibration in vivo for several hours, the appearance of the implanted cells, notably the bead-cytoplasm boundary, was examined by light and electron microscopy of sections and found to be unaffected. After incubation the beads were isolated, briefly rinsed, and their protein contents examined by one- or two-dimensional gel electrophoresis. We show that diffusible proteins can be identified by their inclusion in the pores of the gel filtration beads used and that their approximate sizes can be estimated from the size exclusion values of the specific materials used. The application of this method to important cell biological questions is demonstrated by showing that several "karyophobic proteins," i.e., proteins of the cytosolic fraction which accumulate in the cytoplasm in vivo, are indeed diffusible in the living oocyte and appear with sizes similar to those determined in vitro. This indicates that the nucleo-cytoplasmic distribution of certain diffusible proteins is governed, in addition to size exclusion at nuclear pore complexes and karyophilic "signals," by other, as yet unknown forces. Some possible applications of this method of gel filtration in vivo are discussed.

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