C ells are faced with something of a problem when it comes to assembling ribosomes, the rotund two-tiered factories that produce proteins. Although mature ribosomes reside in the cytoplasm, the large 60 S and small 40 S subunits of which they are composed are assembled from ribosomal RNAs and proteins in a region of the nucleus called the nucleolus. To address the problem of making ribosomes in one place but needing them in another, cells have organized an intricate transport system to ensure that ribosomal proteins are moved into the nucleus, and ribosomal subunits are exported from the nucleus to the cytoplasm. Like traffic wardens directing cars at a busy intersection, a legion of proteins keep the stream of macromolecular traffic—proteins, RNAs, ribosomal subunits—flowing freely through pores in the nuclear membrane, the boundary between nucleus and cytoplasm. According to Johnson and colleagues ([1][1]), there are at least two traffic warden proteins involved in the nuclear export of the 60 S ribosomal subunit: Nmd3p, which carries a putative leucine-rich nuclear export signal, and Crm1p, an export receptor that recognizes this signal. Having established that Nmd3p binds tightly to the yeast 60 S ribosomal subunit in the final stages of its assembly ([2][2]), the Johnson laboratory engineered yeast expressing mutant Nmd3p that lacked the carboxyl terminus containing the nuclear export signal. By tagging the large ribosomal subunit protein L25 with a green fluorescent marker, the authors could track the location of the 60 S subunit in these yeast mutants. In yeast lacking the nuclear export signal, L25 became sequestered in the nucleus (see left figure), whereas in yeast with fully functional Nmd3p, L25 moved into the cytoplasm (see right figure). Thus, 60 S ribosomal subunits continued to be exported from the nucleus as long as Nmd3p retained its nuclear export signal. The authors surmise that Nmd3p is an adapter protein that binds to the 60 S subunit, providing it with a nuclear export signal (such signals are necessary for any protein to leave the nucleus). ![Figure][3] The Crm1p export receptor transports protein cargo carrying a leucine-rich nuclear export signal through nuclear pores by interacting with proteins in the nuclear pore complex. When they inactivated yeast Crm1p with the antibiotic leptomycin B, Johnson and co-workers found that Nmd3p and fluorescent L25 were retained in the nucleus, whereas in leptomycin- insensitive yeast both Nmd3p and L25 moved freely into the cytoplasm. They propose that the 60 S subunit is exported from the nucleus by Crm1p, which binds to the nuclear export signal provided by Nmd3p. Just as traffic on a freeway must pay a toll to pass through a toll booth, nuclear pores exact an energy payment as cargo proteins exit the nucleus. This payment takes the form of conversion of energy-rich guanosine triphosphate (GTP) to guanosine diphosphate through the activity of the small GTPase, Ran. Ran has been identified in many different complexes of cargo proteins and their associated export receptors. In an in vitro assay, the Johnson group has found that Nmd3p, Crm1p and Ran associate together in a complex ([3][4]). The next step will be to examine how this complex interacts with the 60 S ribosomal subunit and what structural rearrangements of the nuclear pore complex are required to enable this large conglomerate to squeeze through the nuclear pores. 1. [↵][5]1. J. Ho, 2. G. Kallstrom, 3. A. W. Johnson , J. Cell Biol. 151, 1057 (2000). [OpenUrl][6][Abstract/FREE Full Text][7] 2. [↵][8]1. J. Ho, 2. G. Kallstrom, 3. A. W. Johnson , RNA 6, 1625 (2000). [OpenUrl][9][Abstract][10] 3. [↵][11]1. J. Ho, 2. G. Kallstrom, 3. A. W. Johnson , unpublished. 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