Selective cleavage of human ACTH, beta-lipotropin, and the N-terminal glycopeptide at pairs of basic residues by IRCM-serine protease 1. Subcellular localization in small and large vesicles

We present a study of the cleavage specificity of IRCM-serine protease 1 from frozen porcine pituitary neurointermediate lobes using polypeptide substrates representing different segments of human pro-opiomelanocortin. Using 125I-labeled ACTH(11-24) and a 125I-labeled model beta-lipotropin (beta-LPH) peptide, the preference of this protease for cleavage C-terminal to the pairs of basic residues Lys-Arg and Lys-Lys was clearly seen. This study was extended to larger unlabeled natural human polypeptides including ACTH(1-39), beta-LPH(1-89), and the N-terminal glycopeptide (1-76), which are known to serve as substrates for further cleavage in vivo. In these substrates IRCM-serine protease 1 cleaved C-terminal to all pairs of basic residues known to be cleaved in vivo. In addition, the enzyme cleaved between two pairs of basic amino acids found in NT(1-76) which are also known to be cleaved in vivo. Many potential "tryptic-like" cleavage sites were not cleaved by the enzyme. However, IRCM-serine protease 1 cleaved C-terminal to Phe-Arg in the three melanocyte-stimulating hormone sequences of pro-opiomelanocortin. In order to better understand the physiological role of IRCM-serine protease 1, differential centrifugation was used to study the subcellular distribution of the enzyme from porcine pituitary anterior lobe homogenates. We present evidence that the active enzyme form, isolated from the subcellular fractions, possesses a similar molecular architecture as the enzyme isolated from frozen tissue (Mr 38,000 catalytic domain linked via disulfide bridge(s) to another polypeptide chain(s) to form an Mr 88,000 monomeric structure). The majority of IRCM-serine protease activity is found to be associated with small vesicles (150,000 X g for 5 h) of as yet undetermined nature. In addition, a latent activity was found to be associated with a 27,000 X g (15 min) pellet containing the majority of mature secretory granules. If IRCM-serine protease 1 participates in prohormone maturation in vivo, we propose a model in which this protease is present in an enzymatically active form in small vesicles, possibly within clathrin-coated structures (prosecretory granules) which are then transformed to mature secretory granules by a process which would also inactivate most of the enzyme.