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MBC in Press, published online ahead of print October 31, 2007
Mol. Biol. Cell 10.1091/mbc.E07-08-0796

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Submitted on August 20, 2007
Revised on October 16, 2007
Accepted on October 19, 2007

The Peripheral Membrane Subunits of the SAM Complex Function Co-dependently in Mitochondrial Outer Membrane Biogenesis

Nickie C. Chan and Trevor Lithgow

Department of Biochemistry and Molecular Biology and Bio21 Molecular Science and Biotechnology Institute, The University of Melbourne, Parkville, Victoria 3010, Australia

Monitoring Editor: Thomas Fox

The SAM complex functions in the assembly of {beta}-barrel proteins into the mitochondrial outer membrane. It is related to the Omp85/YaeT machinery in bacterial outer membranes, but the eukaryotic SAM complex is distinguished by two peripheral subunits, Sam37 and Sam35, that sit on the cytosolic face of the complex. The function of these subunits in {beta}-barrel protein assembly is currently unclear. By screening a library of sam35 mutants, we show that thirteen distinct alleles were each specifically suppressed by overexpression of SAM37. Two of these mutants, sam35–409 and sam35–424, show distinct phenotypes that enable us to distinguish the function of Sam35 from that of Sam37. Sam35 is required in order for the SAM complex to bind outer membrane substrate proteins: destabilisation of Sam35 inhibits substrate binding by Sam50. Sam37 acts later than Sam35, apparently to assist release of substrates from the SAM complex. Very different environments surround bacteria and mitochondria, and we discuss the role of Sam35 and Sam37 in terms of the problems peculiar to mitochondrial protein substrates.

Address correspondence to: Trevor Lithgow (t.lithgow{at}unimelb.edu.au)






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