Lipid membrane consists of two layers – the inner layer and outer layer. Each of them contains specific proteins associated with a certain function. Right-side-out vesicles (ROVs) represent a portion of vesicles with normal orientation of two lipid leaflets. Inside-out vesicles (IOVs) are characterized by the inversed direction of two lipid layers. As a result, in IOVs, the inner layer of lipid membrane is localized on the vesicle surface and interacts with the external environment whereas outer layer of lipid membrane in localized inside and faces the internal content of the IOV vesicle. Fluorescent microscopy reveals that Annexin V is localized in the inner layer of the normal lipid membrane. As a result, in IOVs, Annexin V interacts with the outside layer and can be easily detected using fluorescent microscopy giving intense green fluorescence. ROVs are characterized by typical localization of Annexin V where it interacts with the inner lipid leaflet giving a corresponding fluorescent signal (green speckles).
Therefore, it can be deduced that Annexin V is a lipid-binding protein that interacts with the inner lipid layer of cell membranes.