Plant and Bacterial Metaxin-like Proteins: Novel Proteins Related to Vertebrate Metaxins Involved in Uptake of Nascent Proteins into Mitochondria

The metaxins were originally identified as vertebrate proteins of the outer mitochondrial membrane involved in protein import into mitochondria. Metaxin proteins have also been found in diverse invertebrate phyla. The present study is concerned with examining whether metaxin-like proteins occur in plants and bacteria. Metaxin-like proteins were revealed by their homology with human metaxins and the possession of characteristic GST_Metaxin protein domains. The results demonstrate that metaxin-like proteins exist in plants that include a wide variety of angiosperms, both eudicots and monocots, and other plant groups. Metaxin-like proteins can also be detected in bacteria, particularly in the Proteobacteria phylum, but also in different bacterial phyla. Phylogenetic analysis indicates that plant metaxin-like proteins, bacterial metaxin-like proteins, and vertebrate metaxins form distinct phylogenetic groups, but are related. Metaxin-like proteins, however, are only distantly related to GSTs (glutathione S-transferase proteins). A similar degree of homology is found in aligning the amino acid sequences of plant and bacterial metaxin-like proteins with human metaxins 1, 2, and 3 and other vertebrate metaxins. The amino acid identities range from about 22%-28% for each alignment. The presence of two conserved protein domains, GST_N_Metaxin and GST_C_Metaxin, in both plant and bacterial metaxin-like proteins provides evidence that these proteins are related to the vertebrate and invertebrate metaxins. The metaxin-like proteins have predicted secondary structures that are dominated by alpha-helical segments, like the vertebrate and invertebrate metaxins.