REGULATION OF THE SARCOPLASMIC-RETICULUM CA2+-RELEASE CHANNEL REQUIRES INTACT ANNEXIN-VI

Annexin VI has eight highly conserved repeated domains; all other annexins have four. Diaz-Munoz et al. (J Biol Chem 265:15894, 1990) reported that annexin VI alters the gating properties of the ryanodine-sensitive Ca2+-release channel isolated from sarcoplasmic reticulum. To investigate the domain structure of rat annexin VI (67 kDa calcimedin) required for this channel regulation, various proteolytic digestions were performed. In each case, protease-resistant core polypeptides were produced. Annexin VI was digested with V8 protease and two core polypeptides were purified by Ca2+-dependent phospholipid binding followed by HPLC. The purified fragments were shown to be derived from the N- and C-terminal halves of annexin VI, and demonstrated differential immunoreactivity with monoclonal antibodies to rat annexin VI. While both core polypeptides retained their ability to bind phospholipids in a Ca2+-dependent manner, they did not regulate the sarcoplasmic reticulum Ca2+-release channel as did intact annexin VI.