FIG. 10.

Simplified diagram of Nmd3p and Lsg1p binding to 60S subunits. Nmd3p loads onto the pre-60S subunit in the nucleus, possibly as the nascent subunit emerges from the nucleolus. Nmd3p remains associated with the subunit during export to the cytoplasm. Tif6p, Arx1p, Crm1(Xpo1p), and Ran(Gsp1p) also accompany the subunit during export but are not shown for clarity. In the cytoplasm, Lsg1p likely binds to the newly exported free 60S subunit, and we suggest that it is required for recycling an as-yet-unidentified factor to the nucleus. Both Nmd3p and Lsg1p are released prior to or upon subunit joining. When released, Nmd3p can reenter to the nucleus for another round of subunit export or bind to a recycling free 60S subunit after translation termination. Lsg1p, on the other hand, is restricted to the cytoplasm and, after it is released from a subunit, it can bind to a newly exported 60S subunit or to a recycling free 60S subunit as with Nmd3p.