Maturation of pro-hormone convertases PC2 and PC3 and their interaction with the neuroendocrine protein 7B2
The activation of many pro-hormones occurs through cleavage at pairs of basic residues and is mediated by two serine proteases, PC2 and PC3. Like their substrates, they are also synthesised as inactive precurors (pro-PC2 and pro-PC3). Maturation is autocatalytic and requires removal of the N-terminal pro-peptide. Pro-PC3 matures within the endoplasmic reticulum (ER), whereas maturation of pro-PC2 proceeds within the later compartments of the Golgi network (TGN)/secretory vesicle (SV) and is thought to be regulated by 7B2. In this study the molecular basis for the differences in the maturation location and the interaction with 7B2 was examined by performing domain swap and site directed mutagenesis experiments. The mutant constructs were analysed within an in vitro cell free system. The results suggest that the oxyanion hole residue (Asp310) of pro-PC2 restricts maturation to a late secretory compartment and is important for the interaction with 7B2. Mutation of this residue to resemble that of PC3 (Asp310Ans), altered pro-PC2 maturation from a TGN/SV like environment to an ER like environment. Maturation of pro-PC2, but not pro-PC3, was shown to be inhibited by 7B2. Residue Asp310 of PC2 was necessary to mediate this interaction. When a similar mutant of PC3 was created to resemble PC2 (Asn309Asp), this residue was not sufficient to alter the maturation profile of pro-PC3 nor was it able to confer 7B2 sensitivity. The pro-region of PC2 was sufficient to alter maturation of PC3 from the ER-like compartment to a TGN/SV-like compartment, but was not able to confer 7B2 sensitivity onto PC3. This study also demonstrated that a basic cluster (HHKQQ88) of pro-PC2 was important for delaying maturation to a late compartment and that the residue Phe104, was important for efficient maturation. Mutational analysis of pro-7B2 revealed that a region within residues 1-151 was important for the interaction with pro-PC2.