Tom22 is a multifunctional organizer of the mitochondrial preprotein translocase

Mitochondrial preproteins are imported by a multisubunit translocase of the outer membrane (TOM), including receptor proteins and a general import por e(1-5). The central receptor Tom22 binds preproteins through both its cytos olic domain and its intermembrane space domain(6-10) and is stably associat ed with the channel protein Tom40 (refs 11-13). Here we report the unexpect ed observation that a yeast strain can survive without Tom22, although it i s strongly reduced in growth and the import of mitochondrial proteins. Tom2 2 is a multifunctional protein that is required for the higher-level organi zation of the TOM machinery. In the absence of Tom22, the translocase disso ciates into core complexes, representing the basic import units, but lacks a tight control of channel gating. The single membrane anchor of Tom22 is r equired for a stable interaction between the core complexes, whereas its cy tosolic domain, serves as docking point for the peripheral receptors Tom20 and Tom70. Thus a preprotein translocase can combine receptor functions wit h distinct organizing roles in a multidomain protein.