FAXC and Metaxin Proteins of Fungi and Bacteria: Distinct Categories of Proteins with Shared Structural Features

FAXC (failed axon connections) proteins are shown to be present in fungi and bacteria and to have structural homology to metaxin proteins of vertebrates and invertebrates. Fungal and bacterial FAXC proteins also share structural features with the metaxin-like proteins of plants and protists. But the FAXC and metaxin proteins are different categories of proteins, as seen in the low percentages of identical amino acids in pairwise sequence alignments. The metaxin-like structural features include the conserved protein domains that are characteristic of metaxin proteins. These are the GST_N_Metaxin, GST_C_Metaxin, and Tom37 domains. Another metaxin-like structural feature is a conserved pattern of -helical and {beta}-strand secondary structure. The FAXC proteins of both fungi and bacteria share this pattern of secondary structure with vertebrate and invertebrate metaxin proteins. The predicted 3D structures of fungal and bacterial FAXC proteins revealed a characteristic 4-stranded, {beta}-sheet motif that was also found to characterize metaxin proteins. Phylogenetic analysis demonstrated that fungal and bacterial FAXC proteins are related by evolution to vertebrate metaxin proteins, though more closely related to vertebrate FAXC proteins. The neighboring genes of the FAXC genes of fungi are different than the neighboring genes of vertebrate FAXC and metaxin genes. A high level of protein sequence homology was found for the multiple predicted FAXC proteins of fungi that have multiple FAXC genes, and in comparing the FAXC proteins of different species of bacteria of the same genus.