REFERENCES AND NOTES ___________________________ 1. D. S. McKay et al., Science 273, 924 (1996). 2. H. Y. McSween, Meteoritics 29, 757 (1994). 3. E. Jagoutz, A. Sorowka, J. D. Vogel, H. Wa ¨ nke, ibid., p. 478. 4. R. D. Ash, S. F. Knott, G. Turner, Nature 380, 57 (1996); D. H. Garrison and D. D. Bogard, Meteoritics Planet. Sci. 32, A45 (1997 ); G. Turner, S. F. Knott, R. D. Ash, J. D. Gilmour, Geochim. Cosmochim. Acta 61, 3835 (1997 ). 5. R. H. Carr, M. M. Grady, I. P. Wright, C. T. Pillinger, Nature 314, 248 (1985). 6. I. P. Wright, M. M. Grady, C. T. Pillinger, Geochim. Cosmochim. Acta 52, 917 (1988). 7. R. N. Clayton and T. K. Mayeda, ibid., p. 925. 8. I. P. Wright, M. M. Grady, C. T. Pillinger, ibid. 56, 817 (1992). 9. A. J. T. Jull, C. J. Eastoe, S. Xue, G. F. Herzog, Meteoritics 30, 311 (1995). 10. A. J. T. Jull, C. J. Eastoe, S. Cloudt, J. Geophys. Res. 102, 1663 (1997 ). 11. I. P. Wright, M. M. Grady, C. T. Pillinger, Nature 340, 220 (1989). 12. M. M. Grady, I. P. Wright, C. Douglas, C. T. Pillinger, Meteoritics 29, 469 (1994). 13. K. S. Hutchins, thesis, University of Colorado (1997 ). 14. A. J. T. Jull, D. J. Donahue, T. W. Linick, Geochim. Cosmochim. Acta 53, 2095 (1989). 15. A. J. T. Jull and D. J. Donahue, ibid. 52, 1309 (1988). 16. For a mean composition of 12.7% carbon for ALH84001 carbonates, production of 14 C by spalla- tion of oxygen, by irradiation of this material in a small object in space, must result in about 74 disintegra- tions per minute per kilogram (3.2  10 8 14 C atoms per gram) (9, 10). This calculated 14 C activity corre- sponds to a 14 C/ 12 C ratio (atom/atom) of 5.0  10 -14 or 4.3% of the ratio found in modern, pre- bomb carbon (1950 A.D.). Using the composition of calcite for EETA79001 results in a value of 4% of that in modern carbon (9, 10, 17 ). After 13 and 12 ka (the terrestrial ages of these two meteorites), we would expect the 14 C in carbonates from these two meteorites to have decayed to a level about 0.9% of the modern value. 17. A. J. T. Jull et al., Lunar Planet. Sci. XXIII, 641 (1992). 18. A negligible amount of 14 C could have been pro- duced by spallation reactions in the organic compo- nents of these meteorites. The amount produced by spallation would not produce a significant 14 C/ 12 C ratio. No appreciable 14 C should have been pro- duced while in space in the organic phases through the action of cosmic-ray– generated secondary ther- mal neutrons on 14 N or through thermal neutron capture on 13 C. The reason is that secondary cos- mic-ray neutrons can only become thermalized in- side a large parent meteoroid or in a smaller body with significant water content. Given that all of the martian meteorites so far recovered were irradiated as small objects in space with little water content, very few cosmic-ray– generated neutrons can have been thermalized. M. S. Spergel et al. [Proc. Lunar Planet. Sci. Conf. 16, J. Geophys. Res. 91, 483 (1986)] have shown that for objects of pre-atmo- spheric radius less than 50 g/cm 2 (or approximate- ly 19 kg in mass) that the cosmic-ray–induced ther- mal neutron flux is extremely small and neutron prod- ucts are not detectable. ALH84001 and EETA79001 were much smaller than this size (recovered masses of 2.1 and 7.9 kg, respectively), and thus the thermal neutron flux would be even lower and we can rule out any significant thermal neutron production of 14 C in the organic components of these martian meteor- ites. Consequently, organic material indigenous to ALH84001 or EETA79001 would be expected to have very low 14 C abundance resulting from irradia- tion in space. 19. Carbonaceous material produced in equilibrium with the atmosphere after 1950 A.D. would contain high- er levels of 14 C (up to twice the modern values) be- cause of contamination of the atmosphere by nucle- ar testing [I. Levin et al., Radiocarbon 27, 1 (1985)]. 20. J. L. Gooding, S. J. Wentworth, M. E. Zolensky, Geochim. Cosmochim. Acta 52, 909 (1988). 21. The carbon isotopic compositions are determined as  13 C =   13 C/ 12 C sample  13 C/ 12 C standard -1  10 3 where the standard is Pee Dee belemnite. 22. For each combustion, between 0.25 and 0.40 g of meteorite powder were placed in a cleaned 9-mm Vycor glass tube, which was then evacuated. Ul- trapure oxygen was introduced at a pressure of 0.3 atm. The samples were then combusted at a series of temperature steps each lasting 20 to 30 min. The temperature was controlled by a resis- tance furnace and thermocouple apparatus. After each step, the evolved CO 2 was cryogenically col- lected and cleaned by standard radiocarbon pro- cedures (14, 32). An aliquot of oxygen was added to the cell between each step. The  13 C of each gas was measured with a stable-isotope mass spectrometer, cryogenically recovered from the dual inlet, and then catalytically converted to graphite over iron (33) for 14 C analysis by AMS. The AMS measurements on these graphite targets were made at the University of Arizona AMS Facility (34). Blanks run at each temperature step showed 3  1 g of modern C for each extraction step (35). For our samples, each combustion step yielded between 9 and 79 g of C, with an average sample size of 30 g. 23. M. M. Grady, I. P. Wright, P. K. Swart, C. T. Pillinger Geochim. Cosmochim. Acta 52, 2855 (1988). 24. U. Neupert et al., Meteoritics Planet. Sci. 32, A98 (1997 ). 25. I. P. Wright, M. M. Grady, C. T. Pillinger, Lunar Plan- et. Sci. 28, 1591 (1997 ). 26. C. S. Romanek et al., Nature 372, 655 (1994). 27. J. W. Valley et al., Science 275, 1633 (1997 ). 28. I. Friedman and J. R. O’Niel, U.S. Geol. Surv. Prof. Pap. 440-KK (1977 ). 29. D. Mittlefehldt, Meteoritics 29, 214 (1994); R. P. Har- vey and H. Y. McSween Jr., Nature 382, 49 (1997 ). 30. L. Becker, D. P. Glavin, J. P. Bada, Geochim. Cos- mochim. Acta, 61, 475 (1997 ). 31. J. L. Bada, D. P. Glavin, G. D. McDonald, L. Becker, Science 279, xxx (1998). 32. R. E. Taylor, Radiocarbon Dating: An Archaeological Perspective ( Wiley, New York, 1987 ). 33. P. J. Slota, A. J. T. Jull, T. W. Linick, L. J. Toolin, Radiocarbon 29, 303 (1987 ). 34. D. J. Donahue, T. W. Linick, A. J. T. Jull, ibid. 32, 135 (1990); D. J. Donahue, Int. J. Mass Spectrom. Ion Processes 143, 235 (1995). 35. The blank levels we determined for a series of com- bustion steps gave 3 g (75° to 200°C), 2.5 g (200° to 400°C), and 5.6 g (400° to 600°C) of modern carbon. Other studies on blanks in our laboratory for combustions using this line give a mean blank of 3  1 g of carbon. 36. We are grateful to the Meteorite Working Group for provision of the samples. We wish to thank A. L. Hatheway, D. Biddulph, L. R. Hewitt, and T. E. Lange for technical assistance, and K. Hutchins, G. S. Burr, D. J. Donahue, and C. J. Eastoe for many useful scientific discussions. This work was partly support- ed by NASA grant NAGW-3614 and NSF grant EAR 95-08413. 3 September 1997; accepted 19 November 1997 Import of Mitochondrial Carriers Mediated by Essential Proteins of the Intermembrane Space Carla M. Koehler, Ernst Jarosch, Kostas Tokatlidis, Karl Schmid, Rudolf J. Schweyen, Gottfried Schatz* In order to reach the inner membrane of the mitochondrion, multispanning carrier pro- teins must cross the aqueous intermembrane space. Two essential proteins of that space, Tim10p and Tim12p, were shown to mediate import of multispanning carriers into the inner membrane. Both proteins formed a complex with the inner membrane protein Tim22p. Tim10p readily dissociated from the complex and was required to transport carrier precursors across the outer membrane; Tim12p was firmly bound to Tim22p and mediated the insertion of carriers into the inner membrane. Neither protein was required for protein import into the other mitochondrial compartments. Both proteins may function as intermembrane space chaperones for the highly insoluble carrier proteins. Most proteins imported to mitochondria are synthesized with a cleavable NH 2 -ter- minal targeting sequence and are sorted to their correct intramitochondrial location by the dynamic interaction of distinct trans- port systems in the outer and inner mem- branes (1). The TIM system in the inner membrane consists of two integral mem- brane proteins, Tim17 and Tim23, which make up the inner membrane import chan- nel. Complete translocation into the matrix is coupled to adenosine triphosphate (ATP) hydrolysis and is mediated by Tim44, mHsp70, and GrpE. However, some of the most abundant inner membrane proteins, such as the metabolite carriers, are synthe- sized without a cleavable NH 2 -terminal pre- sequence and therefore do not engage with the Tim23 channel. It has been suggested that import of these proteins is directed by one or more internal targeting signals (2), but the exact mechanism is still poorly de- fined. In the cytosol of the yeast Saccharo- myces cerevisiae, chaperones escort these in- soluble carrier proteins preferentially to the outer membrane receptors Tom37 and Tom70 (3). The carriers then move through the TOM channel in the outer membrane and insert into the inner mem- brane, bypassing the ATP-dependent Tim23 system, which transports proteins C. M. Koehler, K. Tokatlidis, K. Schmid, G. Schatz, Biozentrum, University of Basel, Klingelbergstrasse 70, CH-4056 Basel, Switzerland. E. Jarosch and R. J. Schweyen, Institut fu ¨ r Mikrobiologie und Genetik, University of Vienna, A-1030 Vienna, Austria. * To whom correspondence should be addressed. E-mail: schatz@ubaclu.unibas.ch REPORTS www.sciencemag.org  SCIENCE  VOL. 279  16 JANUARY 1998 369