MEMBRANE-MEDIATED ASSEMBLY OF ANNEXINS STUDIED BY SITE-DIRECTED SPIN-LABELING

Annexins are soluble proteins that bind to membranes in the presence o f Ca2+. Crystal structures have been determined for some soluble forms , but little is known about the important membrane-bound state. We emp loyed site-directed spin labeling to demonstrate that 1) annexin XII a ssumes a trimer configuration similar to the crystal structure when bo und to bilayers under physiological conditions; 2) trimer assembly on bilayers is remarkably rapid, occurring on a millisecond time scale, w hereas subunit exchange requires hours; and 3) different annexins can mix to form heterotrimers. The rapid assembly and heterotrimer formati on have important implications concerning the cellular functions of an nexins.