The prohormone convertase enzyme 2 (PC2) is essential for processing pro-islet amyloid polypeptide at the NH2-terminal cleavage site

(proIAPP), the precursor of the beta -cell peptide islet amyloid polypeptid e (IAPP) (amylin), has been implicated in islet amyloid formation in type 2 diabetes. The prohormone convertase enzymes PC3 (also known as PC1) and PC 2 are localized to beta -cell secretory granules with proIAPP and proinsuli n and are responsible for proinsulin processing. To determine whether PC2 m ight be essential for proIAPP processing, we performed Western blot analysi s of freshly isolated islets from normal mice and mice lacking active PC2. As expected, the primary species of LAPP immunoreactivity in islets from wi ld-type mice was fully processed (4-kDa) LAPP, with only small amounts of t he 8-kDa precursor (unprocessed proIAPP) present. Islets from heterozygous PC2 null mice were identical to wild-type animals, suggesting that half the normal complement of PC2 is sufficient for normal proIAPP processing. By c ontrast, in islets from homozygous PC2 null mice, the predominant IAPP-immu noreactive form was of intermediate size (similar to6 kDa), with no detecta ble mature IAPP and slightly elevated amounts of the 8-kDa precursor form p resent. Thus, in the absence of PC2, proIAPP processing appears to be block ed at the level of a proIAPP conversion intermediate. Immunofluorescence of pancreas sections and immunoblotting using antisera raised to the NH2- and COOH-terminal flanking regions of mouse proIAPP demonstrated that the 6-kD a intermediate form was an NH2-terminally extended proIAPP conversion inter mediate (processed only at the COOH-terminus). These data indicate that PC2 is essential for processing of proIAPP at the NH2-terminal cleavage site i n vivo and that PC3 is likely only capable of processing proIAPP at the COO H-terminal cleavage site.