Fusogenic Activity of Annexins A1 and A5

Abstract

Annexins are a family of proteins that bind to anionic phospholipids in a calcium-dependent manner and have been implicated in a number of cell biological processes including intracellular fusion. Recently we found that extracellular annexins (A1 and A5) play important roles in myoblast fusion. In this study we explored fusogenic activity of these proteins in the absence of other proteins and found both A1 and A5 to aggregate phophatidylserine-containing liposomes and to induce lipid mixing between these liposomes. Annexins A1 and A5 drive fusion beyond early hemifusion stage, as evidenced by lipid mixing between the inner leaflets of liposomal membranes. The annexin-mediated vesicle aggregation and lipid mixing depended on the presence of calcium, anionic lipids and were inhibited by specific antibodies and, in the case of annexin A1, by a synthetic peptide derived from its N-terminal domain. Annexins A1 and A5 applied together were more effective in inducing fusion than either of the proteins on its own. This synergistic activity can be important in fusion between myoblasts since both of these proteins are present at myoblast surface under fusion conditions. Our results support the hypothesis that annexins directly mediate myoblast fusion by mechanisms similar to those discussed for viral and intracellular protein fusogens rather than regulate cell fusion via other proteins.