A novel function for the 90 kDa heat-shock protein (Hsp90): facilitating nuclear export of 60 S ribosomal subunits.

Ribosomal subunits are assembled in the nucleus. and mature 40 S and 60 S subunits are exported stoichiometrically into the cytoplasm. The nuclear export of ribosomal subunits is a unidirectional, saturable and energy-dependent process. An in citro assay for the nuclear export of 60 S ribosomal subunits involves the use of resealed nuclear envelopes. The export of ribosomal subunits from resealed nuclear envelopes is enhanced by cytoplasmic proteins. Here we present evidence that the export-promoting activity was due to the cytoplasmic 90 kDa heat-shock protein (Hsp90). Isolated. purified Hsp90 vastly enhanced the export of 60 S ribosomal subunits from resealed nuclear envelopes. while inhibition of Hsp90 function. either with the Hsp90-binding drug geldanamycin or with anti-Hsp90 antibodies, resulted in reduced release of 60 S ribosomal subunits. To confirm these findings under in vivo conditions. corresponding experiments were performed with Xenopus oocytes using micro-injection techniques, the results obtained confirmed the findings obtained with resealed nuclear envelopes. These findings suggest that Hsp90 facilitates the nuclear export of 60 S ribosomal subunits, probably by chaperoning protein interactions during the export process.