The transport of precursor proteins across the outer membrane of mitochondria is mediated by the translocase of the outer membrane (TOM), a multicomponent proteic complex of high molecular weight that spans the membrane, forming a channel-like corridor. TOM also sorts preproteins destined for the inside of the mitochondrion from those to be inserted in the outer and inner membranes and assists in assembling these membrane proteins. In the July 31 Nature, Nils Wiedemann and colleagues at the Universität Freiburg reexamined the sorting and assembly of proteins in an organellar membrane and suggest that it may be more complicated than originally thought (Nature, 424:565-571, July 31, 2003).

Wiedemann et al. focused on studying the function of Mas37, a protein present in the outer mitochondrial membrane and believed to be involved in protein import, although it not as part of the TOM complex. They tracked the assembly of...

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