Tom40, the import channel of the mitochondrial outer membrane, plays an active role in sorting imported proteins.

The Translocase of the Outer Mitochondrial membrane (TOM complex) is centred on a channel, created by Tom40, serving as the only means of entry for proteins into the mitochondrion. Proteins destined for internal mitochondrial compartments interact subsequently with one of the two distinct protein Translocases of the Inner Mitochondrial membrane (TIM23 and TIM54 complexes) or follow specialized paths into the intermembrane space. To investigate the sorting of precursor proteins to these various sub-mitochondrial compartments, we created a library of tom40 mutants and screened for alleles selectively corrupt in protein sorting.

Bipartite signals mediate subcellular targeting of tail-anchored membrane proteins in Saccharomyces cerevisiae.

Tail-anchored proteins have an NH(2)-terminal cytosolic domain anchored to intracellular membranes by a single, COOH-terminal, transmembrane segment. Sequence analysis identified 55 tail-anchored proteins in Saccharomyces cerevisiae, with several novel proteins, including Prm3, which we find is required for karyogamy and is tail-anchored in the nuclear envelope. A total of six tail-anchored proteins are present in the mitochondrial outer membrane and have relatively hydrophilic transmembrane segments that serve as targeting signals.

A conserved proline residue is present in the transmembrane-spanning domain of Tom7 and other tail-anchored protein subunits of the TOM translocase.

The TOM translocase consists of several integral membrane proteins organised around the channel forming protein Tom40. Here we show that one of these protein subunits, Tom7, is a tail-anchored protein. The carboxy-terminal 33 amino acids of Tom7 contain the information for targeting the protein to the mitochondrial outer membrane, and a conserved proline residue within the transmembrane segment is required for efficient targeting of Tom7 to the outer membrane.