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 com prises an assembly of seven polypeptide subunits designated alpha, beta, be ta', gamma, delta, epsilon, and zeta COP. Rooted phylogenetic trees constru cted from the full-length cDNA and amino acid sequences of 49 COP entities in different eukaryotes from yeast to man generally revealed striking conse rvation of each subunit through evolution. Both nucleotide and protein tree s displayed close relationships between alpha and beta' subunits, between b eta and gamma subunits, and between delta and zeta subunits, implying evolu tion from common ancestors as well as functional similarity. Interestingly, although 6 out of 7 epsilon -COP genes appeared to be grouped and related to the beta -COP genes, 4 out of 7 epsilon -COP gene products clustered wit h other groups of other COP subunit proteins. A 5' coding segment of the ma rine alpha -COP gene was amplified by RT-PCR and cycle-sequenced. The parti al predicted amino acid sequence of this murine homolog was exactly identic al to the human and bovine counterparts. Of particular significance was the complete identity, of the first 25 and 35 N-terminal residues which consti tute the gastrointestinal hormone xenin and its precursor proxenin, thus em phasizing their strict evolutionary conservation and alluding to their phys iological importance.