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Originally published as MBC in Press, 10.1091/mbc.E07-08-0796 on October 31, 2007

Vol. 19, Issue 1, 126-136, January 2008

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The Peripheral Membrane Subunits of the SAM Complex Function Codependently 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

Submitted August 20, 2007; Revised October 16, 2007; Accepted October 19, 2007
Monitoring Editor: Thomas Fox

The sorting and assembly machinery (SAM) complex functions in the assembly of β-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 β-barrel protein assembly is currently unclear. By screening a library of sam35 mutants, we show that 13 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 for the SAM complex to bind outer membrane substrate proteins: destabilization 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.

This was published online ahead of print in MBC in Press (http://www.molbiolcell.org/cgi/doi/10.1091/mbc.E07-08-0796) on October 31, 2007.

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






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