Metallothionein in Yak Characterization of Metallothionein-III in Yak (Bos grunniens)

Metallothinonein-III (MT-III) as a new member of the Metallothionein (MT) family has specific physiological effects different from known MT-I and MT-II. In this study, the yak MT-III gene coding region was amplified and cloned by RT-PCR from brain tissue of yak using YMT-IIISP1 and YMT-IIISP2 as specific primers. The isolated cDNA sequence of MT-III was 207 bp in length (Genbank accession, NO, DQ323545) and was subjected to BLASTn searching in NCBI. Results of the search indicate the nucleotide sequences of yak share 98, 97, 96, 92, 91, 90, 89, 88 and 86% sequence similarity with cattle, milk goat, pig, sheep, chimpanzee, human, dog and house mouse, respectively. Comparing homologies of MT-III sequences with MT-I and MT-II in yak, we found 69 and 67% homologies, respectively. The MT-III protein was composed of 68 amino acids, including 19 cysteines, similar to the number of cysteines of sheep but not human and mouse which lack the conserved ninth cysteine and have no aromatic amino acids. There were conserved motifs of MTs, such as C-X-C, C-C-X-C-C, C-X-X-C and KKS and specific motifs including MDPE, CPCP in MT-III. This conservation of motifs suggests a conservation of MT-III in molecular evolution. The MT-III in yak had no signal peptide and represented a form of cytoplasmic protein similar to MT-I/II. There were few sheets in secondary protein structures, obvious helices in 39-46th AA and mainly irregular curling in the 2D-structure of MT-III protein. The lack of the conserved ninth cysteine in yak MT-III merits further research.