tRNA transport from the nucleus in a eukaryotic cell: carrier-mediated translocation process.

Abstract

The mechanism by which a tRNA molecule is delivered from the nucleus of a cell to the cytoplasm was studied in the Xenopus laevis oocyte utilizing nuclear microinjection and manual microdissection techniques. tRNA nuclear transport in this cell resembles a carrier-mediated translocation process rather than diffusion through a simple pore or channel. tRNA transport is saturable by tRNA, with a maximal rate measured to be .apprx. 190 .times. 107 molecules/min per nucleus (21.degree. C) in the mature oocyte. Competitive inhibition between 2 different tRNA species can be demonstrated, suggesting that many tRNA species share a common carrier system. tRNA nuclear transport is sharply dependent on temperature, with an optimal rate observed at 31.degree. C. A single G-to-U substitution at position 57 in the vertebrate .**GRAPHIC**. molecule reduces the trasport rate of this tRNA by a factor of about 20, implicating this highly conserved region of the tRNA molecule (loop IV) as critical for recognition by the transport mechanism. On morphologic grounds it is proposed that ribosome-like components surrounding the nuclear pore may function as the tRNA translocation motor. The tRNA nuclear transport mechanism represents a distinctly eukaryotic process and a site of potential control over cell growth and proliferation.