METAXIN IS A COMPONENT OF A PREPROTEIN IMPORT COMPLEX IN THE OUTER-MEMBRANE OF THE MAMMALIAN MITOCHONDRION

Metaxin, a novel gene located between the glucocerebrosidase and throm bospondin 3 genes in the mouse, is essential for survival of the posti mplantation mouse embryo, In this study, the subcellular location, dom ain structure, and biochemical function of metaxin mere 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 m etaxin cDNA was transfected into COS cells, immunofluorescence stainin g demonstrated that the protein is located in mitochondria. Metaxin co ntains a putative mitochondrial outer membrane signal anchor domain at its C terminus, and a truncated form of metaxin lacking this signal a nchor domain had a reduced association with mitochondria. In addition, metaxin was highly susceptible to proteases in intact mitochondria, W e there fore conclude that metaxin is a mitochondrial protein that ext ends 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 m itochondrial preprotein translocation apparatus in Saccharomyces cerev isiae, but important structural differ ences, including apparently dif ferent mechanisms of targeting to membranes, also exist between the tw o proteins, Given the similar subcellular locations of metaxin and Tom 37, the possible role of metaxin in mitochondrial preprotein import wa s investigated, Antibodies against metaxin, when preincubated with mit ochondria, partially inhibited the uptake of radiolabeled preadrenodox in into mitochondria, Metaxin is therefore the second mammalian compon ent 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 le thal phenotype of mice lacking metaxin demonstrates that efficient imp ort of proteins into mitochondria is crucial for cellular survival. Th e characterization of metaxin provides an opportunity to elucidate sim ilarities 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.