Analyzing the evolution of β-endorphin post-translational processing events: Studies on reptiles

Abstract

In many cartilaginous fishes, most ray-finned fishes, lungfishes, and amphibians, the post-translational processing of POMC includes the monobasic cleavage of β-endorphin to yield an opioid that is eight to ten amino acids in length. The amino acid motif within the β-endorphin sequence required for a monobasic cleavage event is -E-R-(S/G)-Q-. Mammals and birds lack this motif and as a result β-endorphin(1–8) is a not an end-product in either group. Since both mammals and birds were derived from ancestors with reptilian origins, an analysis of β-endorphin sequences from extant groups of reptiles should provide insights into the manner in which β-endorphin post-translational processing mechanisms have evolved in amniotes. To this end a POMC cDNA was cloned from the pituitary of the turtle, Chrysemys scripta. The β-endorphin sequence in this species was compared to other reptile β-endorphin sequences (i.e., Chinese soft shell turtle and gecko) and to known bird and mammal sequences. This analysis indicated that either the loss of the arginine residue at the cleavage site (the two turtle species, chick, and human) or a substitution at the glutamine position in the consensus sequence (gecko and ostrich) would account for the loss of the monobasic cleavage reaction in that species. Since amphibians are capable of performing the β-endorphin monobasic reaction, it would appear that the amino acid substitutions that eliminated this post-translational process event in reptilian-related tetrapods must have occurred in the ancestral amniotes.