Molecular cloning and phylogenetic analysis of AmphiUbf80, a new member of ubiquitin family from the amphioxus Branchiostoma belcheri tsingtauense

摘要

An amphioxus cDNA, AmphiUbf80, encoding ubiquitin (Ub)/ribosomal fused protein S27a, was isolated from the gut cDNA library of Branchiostoma belcheri tsingtauense. The deduced N-terminal 76 amino acids of the protein were 100% identical to those of human and rat Ub, and the carboxyl extension protein (CEP) sequences were also highly similar to those of the homologous proteins of human, rat and other species. Alignment data of the CEP sequences from different animals showed that AmphiUbf80 was not only similar, to vertebrate Ubf80, but also to invertebrate Ubf80. In addition, its C-terminal sequences, 69-CLTFVYNK-76, shared both the vertebrate-specific Ubf80 consensus (69-CLTYCFNK-76) and the invertebrate counterpart sequences. Phylogenetic analysis based on the amino acids of the Ub/ribosomal fused proteins of different animals well reflected the established phylogeny of the chosen organisms; amphioxus was branched off from chordates and formed a separate cluster with an intermediate placement between invertebrates and vertebrates in the tree. However, amphioxus Ubf80 also showed some unique characters, such as its amino acid residues at sites 39, 54, 63 and 74 in the CEP. It appears that amphioxus not only represents an organism transitional from invertebrates to vertebrates in phylogeny, but also a possible diverged form radiated from the chordate ancestor at early chordate evolution. The possibility that some mutations in Ub genes have taken place in different lines early at the divergence of plants and animals is also discussed.