Metaxin, a novel gene located between the glucocerebrosidase and thrombospondin 3 genes in the mouse, is essential for survival of the postimplantation mouse embryo. In this study, the subcellular location, domain structure, and biochemical function of metaxin were investigated. Anti-recombinant metaxin antibodies recognized 35-and 70-kDa proteins in mitochondria from various tissues; the 35-kDa protein is consistent in size with the predicted translation product of metaxin cDNA. When metaxin cDNA was transfected into COS cells, immunofluorescence staining demonstrated that the protein is located in mitochondria. Metaxin contains a putative mitochondrial outer membrane signal anchor domain at its C terminus, and a truncated form of metaxin lacking this signal anchor domain had a reduced association with mitochondria. In addition, metaxin was highly susceptible to proteases in intact mitochondria. We therefore conclude that metaxin is a mitochondrial protein that extends into the cytosol while anchored into the outer membrane at its C terminus. In its N-terminal region, metaxin shows significant sequence identity to Tom37, a component of the outer membrane portion of the mitochondrial preprotein translocation apparatus in Saccharomyces cerevisiae, but important structural differences, including apparently different mechanisms of targeting to membranes, also exist between the two proteins. Given the similar subcellular locations of metaxin and Tom37, the possible role of metaxin in mitochondrial preprotein import was investigated. Antibodies against metaxin, when preincubated with mitochondria, partially inhibited the uptake of radiolabeled preadrenodoxin into mitochondria. Metaxin is therefore the second mammalian component of the protein translocation apparatus of the mitochondrial outer membrane to be characterized at the molecular level and the first for which an inherited mutation has been described. The early embryonic lethal phenotype of mice lacking metaxin demonstrates that efficient import of proteins into mitochondria is crucial for cellular survival. The characterization of metaxin provides an opportunity to elucidate similarities and possible differences in the mechanisms of protein import between fungi and mammals and in the phenotypes of fungi and mammals lacking mitochondrial import receptors.During studies of thrombospondin 3 (Thbs3), which encodes an extracellular matrix protein (2-4), and glucocerebrosidase (Gba), which encodes a lysosomal enzyme (5), a novel gene, termed metaxin (Mtx), was found to span the interval between Thbs3 and Gba on chromosome 3E3-F1 in the mouse (6). Mtx and Thbs3 are situated in a head-to-head orientation, with the translation start sites 1.4 kilobase pairs apart. Gba and Mtx lie in a tail-totail orientation, with the polyadenylation sites spaced 431 base pairs apart. The Mtx promoter resembles the promoters for housekeeping genes in that it is TATA-less, GC-rich, and regulated by tandem Sp1 elements within 300 base pairs of the transcription start site (7). Although ...