The Journal of Cell Biology, Vol 106, 1435-1444, Copyright © 1988 by The Rockefeller University Press
The function of the nuclear envelope in nuclear protein accumulation
FJ Zimmer, C Dreyer and P Hausen
Max-Planck-Institut fur Entwicklungsbiologie, Tubingen, Federal Republic of Germany.
The mechanism by which proteins accumulate in the cell nucleus is not yet
known. Two alternative mechanisms are discussed here: (a) selective
unidirectional entry of karyophilic proteins through the nuclear pores, and
(b) free diffusion of all proteins through the nuclear pores and specific
binding of nuclear proteins to nondiffusible components of the nucleoplasm.
We present experiments designed to distinguish between these alternatives.
After mechanical injury of the Xenopus oocyte nuclear envelope, nuclear
proteins were detected in the cytoplasm by immunohistochemical methods. In
a second approach, nuclei from X. borealis oocytes were isolated under oil,
the nuclear envelopes were removed, and the pure nucleoplasm was injected
into the vegetal pole of X. laevis oocytes. With immunohistochemical
methods, it was found that each of five nuclear proteins rapidly diffuses
out of the injected nucleoplasm into the surrounding cytoplasm. The
subsequent transport and accumulation in the intact host nucleus could be
shown for the nuclear protein N1 with the aid of a species-specific mAb
that reacts only with X. borealis N1. Purified and iodinated nucleoplasmin
was injected into the cytoplasm of Xenopus oocytes and its uptake into the
nucleus was studied by biochemical methods.
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