Invertebrate FAXC and Metaxin Proteins: Related Proteins with Distinct Features

ABSTRACT Structural features of FAXC proteins of invertebrates are examined in this report to further establish that invertebrate FAXCs are metaxin-like proteins. Invertebrate phyla that receive the most attention are Mollusca, Cnidaria, and Arthropoda. The results demonstrate that invertebrate FAXC proteins have structural homology to vertebrate and invertebrate metaxin proteins. However, pairwise alignments of amino acid sequences revealed only low percentages of amino acid identities, indicating that FAXC and metaxin proteins are distinct categories of proteins. Among the important criteria in identifying invertebrate FAXC proteins as metaxin-like was the presence of conserved protein domains: GST_N_Metaxin, GST_C_Metaxin, and Tom37. Another metaxin-like feature found with invertebrate FAXC proteins is a characteristic pattern of α-helical and β-strand segments. The four β-strand segments form, in three dimensions, a planar β-sheet motif that is a distinguishing characteristic of FAXC and metaxin proteins. Evolutionary relationships between FAXCs and metaxins were uncovered by phylogenetic analysis. Distinct but related evolutionary groups were found for invertebrate FAXCs and metaxins, as well as vertebrate FAXCs and metaxins. Invertebrate species can have multiple FAXC genes, and more than ten FAXC genes is not uncommon. The genes that are adjacent to different FAXC genes in the same invertebrate are typically not the same. Therefore, the genomic regions are generally not highly conserved for multiple FAXC genes. But, for invertebrates with a single FAXC gene, the same neighboring genes may be found for the FAXC genes of different but taxonomically related invertebrates. Honey bees and bumblebees are examples. For invertebrates with multiple FAXC genes, several of the genes may be in close proximity in the same genomic region, forming a cluster of FAXC genes.