OCEAN SCIENCE: The Many Shades of Ocean Blue

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  • DOI: 10.1126/science.1092704, 1514 (2003);302 Science

    Herv Claustre and Stphane MaritorenaThe Many Shades of Ocean Blue

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    portin- for FG nucleoporins by altering therelative positions of HEAT repeats 5 and 6 (7,8); this change also facilitates cargo release.

    Crystallography has defined the interac-tion interfaces between importin- and manyof its binding partners. FG nucleoporins bindon the outer, convex surface of importin- toa primary site between HEAT repeats 5 and6 and to several secondary sites, includingone near the carboxyl terminus (7, 8, 15).However, the remainder of importin-sbinding partners interact primarily with theinner, concave face. RanGTP binds to HEATrepeats 1 to 8 at the amino terminus of im-portin- (4), whereas the mainly helical IBBdomain binds over an extensive area of theconcave surface that encompasses HEAT re-peats 7 to 19 (3). The interface between IBBand importin- exhibits an extended networkof interactions involving more than 40 elec-trostatic, hydrophobic, and van der Waalscontacts. Tryptophan residues (especiallytryptophan-864) in importin- help to stabi-lize the helical conformation of the IBB do-main and so ensure specific, high-affinitybinding (16). By contrast, the NLS of PTHrP(parathyroid hormonerelated protein) bindsat the importin- amino terminus (HEAT re-peats 2 to 6) and overlaps with the RanGTPbinding site (9). This NLS adopts primarily a-sheet conformation, and its interaction isreminiscent of the way classic NLSs bind toimportin- (6). Although the binding sitesfor the PTHrP-NLS and IBB domain on im-portin- partly overlap, the molecular archi-tecture of their sites is different and both canbind to importin- simultaneously (9).

    SREBP-2 lacks a classic NLS and, as Leeet al. (10) describe, binds to importin- via itsdimeric helix-loop-helix zipper (HLHZ) do-main. As the investigators show, the crystalstructure of the importin-:SREBP-2 com-plex reveals a remarkable new twist in the waythat importin- recognizes its cargo. In this

    complex, the dimeric HLHZ domain isgripped primarily by the long helices ofHEAT repeats 7 and 17 that move like chop-sticks to pick up the cargo. In contrast to theIBB domain helix, the helices of theHLHZ domain are perpendicular to the cen-tral axis of the importin- superhelix. To ac-commodate this interaction, importin-moves the long helices of HEAT repeats 7 and17 and adopts a more twisted and open con-formation. The conformational change intro-duced in this way is dramatic, with the amino-terminal region moving by 20 (a rotation of22). In contrast to the PTHrP-NLS and theIBB domain, the interaction between im-portin- and the HLHZ domain of SREBP-2is mediated primarily by hydrophobic interac-tions; charge complementation is of less im-portance. Lee et al. also note the presence ofa possible structural repeat in the sequence ofimportin- that offers a convenient way forimportin- to bind to dimeric cargo proteinsand perhaps also to FG nucleoporins (15).

    Overall, the remarkable conformationalvariability of importin-:cargo complexesimplies that unbound free importin- is flex-ible and so can adapt its shape to recognize avariety of different partners. Such flexibilityis consistent with the greater susceptibility offree versus bound importin- to proteolysis(3). Although high-resolution structures ofdifferent binding states have not yet been ob-tained, several homologs of importin- prob-ably also provide an analogous flexible scaf-fold that facilitates versatility in substraterecognition (14). By contrast, although im-portin- interacts with a range of substrates,these interactions do not appear to involvemajor conformational changes, even whenthey accelerate the rate of cargo release (17).

    That importin- must bind to a remarkablerange of different proteins to carry out both itsnuclear trafficking and nuclear assembly tasksexerts extraordinary demands on molecular

    recognition capabilities. Such demands aremade even more pressing by the necessity forimportin- to bind to and release many of itspartners in a spatially and temporally definedsequence. Molecular recognition usually in-volves matching of complementary interfacesbetween interacting proteins, and it is not un-common for relatively small structural alter-ations to be present at the interface associatedwith binding, or for domains of molecules torotate around a hinge. However, the confor-mational changes observed when importin-binds to its partners, particularly when it bindsto SREBP-2, involve an unusually large dis-tortion. Although stacking of 19 tandemHEAT repeats generates an extensive interac-tion surface that enables importin- to bind toa wide range of different molecules, importin- supplements this ability with a remarkabledegree of conformational flexibility. In thisway, it is able to greatly increase its versatilityin recognizing and releasing its different bind-ing partners. As more crystal structures of im-portin- bound to different cargo proteins be-come available, it will be fascinating to seehow the interplay of surface structure andconformational dynamics is exploited to ac-complish a variety of different tasks.

    References1. K. Weis, Cell 112, 441 (2003).2. Y. M. Chook, G. Blobel, Nature 399, 230 (1999).3. G. Cingolani et al., Nature 399, 221 (1999).4. I. R. Vetter et al., Cell 97, 635 (1999).5. Y. M. Chook, G. Blobel, Curr. Opin. Struct. Biol. 11, 703

    (2001).6. E. Conti, E. Izaurralde, Curr. Opin. Cell Biol. 13, 310 (2001).7. R. Bayliss et al., Cell 102, 99 (2000).8. R. Bayliss et al., J. Biol. Chem. 277, 50597 (2002).9. G. Cingolani et al., Mol. Cell 10, 1345 (2003).

    10. S. J. Lee et al., Science 302, 1571 (2003).11. B. Fahrenkrog, U. Aebi, Nature Rev. Mol. Cell Biol. 4,

    757 (2003).12. C. Zhang et al., Curr. Biol. 12, 498 (2002).13. A. Harel et al., Mol. Biol. Cell 14, 4387 (2003).14. N. Fukuhara et al., J. Biol. Chem. M309112200 (2003).15. J. Bednenko et al., J. Cell Biol. 162, 391 (2003).16. C. Koerner et al., J. Biol. Chem. 278, 16216 (2003).17. Y. Matsuura et al., EMBO J. 22, 5358 (2003).

    In satellite data over the South Pacific gyre,Earths largest oceanic desert (see the fig-ure), Dandonneau et al. [page 1548 of this

    issue (1)] have detected areas that are green-er than the surrounding deep blue waters.They suggest that these hotspots are not

    vegetal oases, but rather result from residuesand by-products of marine life accumulatedby physical processes. This detrital materialwould be incorrectly interpreted as phyto-plankton biomass from satellite ocean colordata. The study provides further evidence thatthe current paradigmthe greener the openocean waters, the higher their phytoplanktoncontenthas its limitations.

    For nearly 25 years, ocean color has beenused as a proxy for algal biomass. Empiricalalgorithms (2) for mapping the global phy-toplankton distribution from space rely on

    mean statistical relationships between thechlorophyll a concentration [Chla] (a proxyfor phytoplankton biomass) and the blue-to-green (B/G) reflectance ratio (an optical in-dex for water color). The resulting [Chla]maps are used to estimate the contributionof phytoplankton to global CO2 fixation (3)and investigate long-term changes in vege-tal biomass (4).

    However, there is increasing evidencethat the simple empirical relationship be-tween [Chla] and B/G ratio does not alwayshold. Which optically active componentscause the