THE NUCLEAR IMPORT FACTOR P10 REGULATES THE FUNCTIONAL SIZE OF THE NUCLEAR PORE COMPLEX DURING OOGENESIS

Previtellogenic, stage-1 Xenopus oocytes produce mainly 5S and tRNA, w hereas vitellogenic oocytes, stages 2-6, synthesize predominantly 18S and 28S rRNA. Using nucleoplasmin-coated gold as a transport substrate , it was determined that the shift in synthesis from small to large RN As during oogenesis is accompanied by an increase in both the rates of signal-mediated nuclear import and the functional size of nuclear por es. It was observed that, despite the reduction in transport capacity, gold still accumulated at the cytoplasmic surface of the pores in sta ge-1 oocytes. This suggested that transport in these cells is limited by translocation factors, rather than by cytoplasmic binding factors. Analysis of extracts prepared from stage-1 and vitellogenic oocytes re vealed that the transport factor p10 is more abundant in stage-1 cells . Microinjection of purified p10 into stage-2 oocytes reduced the nucl ear import of large gold particles to the level observed in stage-1 ce lls. It is concluded that p10 can modulate transport through the pores by regulating the functional size of the central transporter element.