Phylogenetic Relationships of the Seven Coat Protein Subunits of the Coatomer Complex, and Comparative Sequence Analysis of Murine Xenin and Proxenin

The coatomer complex is involved in intracellular protein transport and comprises an assembly of seven polypeptide subunits designated ?, ?, ??, ?, ?, ?, and ? COP. Rooted phylogenetic trees constructed from the full-length cDNA and amino acid sequences of 49 COP entities in different eukaryotes from yeast to man generally revealed striking conservation of each subunit through evolution. Both nucleotide and protein trees displayed close relationships between ? and ?? subunits, between ? and ? subunits, and between ? and ? subunits, implying evolution from common ancestors as well as functional similarity. Interestingly, although 6 out of 7 ?-COP genes appeared to be grouped and related to the ?-COP genes, 4 out of 7 ?-COP gene products clustered with other groups of other COP subunit proteins. A 5? coding segment of the murine ?-COP gene was amplified by RT-PCR and cycle-sequenced. The partial predicted amino acid sequence of this murine homolog was exactly identical to the human and bovine counterparts. Of particular significance was the complete identity of the first 25 and 35 N-terminal residues which constitute the gastrointestinal hormone xenin and its precursor proxenin, thus emphasizing their strict evolutionary conservation and alluding to their physiological importance.