Structure and expression of Xenopusprohormone convertase PC2

The multifunctional prohormone, proopiomelanocortin (POMC), is processed in the melanotrope cells of the pituitary pars intermedia at pairs of basic amino acid residues to give a number of peptides, including α-melanophore-stimulating hormone (α-MSH). This hormone causes skin darkening in amphibians during background adaptation. Here we report the complete structure of Xenopus laevisprohormone convertase PC2, the enzyme thought to be responsible for processing of POMC to α-MSH. A comparative structural analysis revealed an overall amino acid sequence identity of 85–87% between XenopusPC2 and its mammalian counterparts, with the lowest degree of identity in the signal peptide sequence (28–36%) and the region amino-terminal to the catalytic domain (59–60%). The occurrence of a second, structurally different PC2 protein reflects the expression of two XenopusPC2 genes. The expression pattern of PC2 in the Xenopuspituitary gland of black- and white-adapted animals was found to be similar to that of POMC, namely high expression in active melanotrope cells of black animals. This observation is in line with a physiological role for PC2 in processing POMC to α-MSH.