The disulfide-bonded loop of chromogranin B mediates membrane binding and directs sorting from the trans-Golgi network to secretory granules

The disulfide-bonded loop of chromogranin B (CgB), a regulated secretory pr otein with widespread distribution in neuroendocrine cells, is known to be essential for the sorting of CgB from the trans-Golgi network (TGN) to imma ture secretory granules. Here we show that this loop, when fused to the con stitutively secreted protein al-antitrypsin (AT), is sufficient to direct t he fusion protein to secretory granules. Importantly, the sorting efficienc y of the AT reporter protein bearing two loops (E2/3-AT-E2/3) is much highe r compared with that of AT with a single disulfide-bonded loop. In contrast to endogenous CgB, E2/3-AT-E2/3 does not undergo Ca2+/pH-dependent aggrega tion in the TGN. Furthermore, the disulfide-bonded loop of CgB mediates mem brane binding in the TGN and does so with 5-fold higher efficiency if two l oops are present on the reporter protein. The latter finding supports the c oncept that under physiological conditions, aggregates of CgB are the sorte d units of cargo which have multiple loops on their surface leading to high membrane binding and sorting efficiency of CgB in the TGN.