Endo- and exovesiculation and the structure of the human red cell membrane

Experiments were designed to study functional associations of proteins in human red cell membranes as the membranes are induced to undergo the critical membrane events of invagination or evagination followed by constriction and fusion. Three examples were chosen for study: the inside-out vesicle (IOV) produced in white ghosts by hypotonic removal of cytoskeletal proteins; the endocytic vacuole produced in white ghosts by incubation with Mg-adenosine triphosphate; and the exocytic vesicle produced by metabolic depletion of intact red blood cells. The resulting particles were harvested, and analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis for membrane protein content and by enzymic analysis to detect the presence or absence of the exofacial enzyme acetylcholinesterase (ACE), the cytosol facing enzyme glyceraldehyde phosphate dehydrogenase (GAPD), and the integral protein adenosine triphosphatases (ATPases). Each of the vesicles is variably depleted of spectrin and actin, and each retains the exofacial enzyme ACE as well as Mg-ATPase. These findings suggest that there must be local partial depletion of cytoskeletal proteins before invagination or evagination occurs and that in each case part of the exoface of the membrane containing ACE is carried along into the resulting vesicle. The two forms of endocytosis differ with regard to their ATPase content with the energized endocytic vacuole retaining Ca-Mg-ATPase and actin-activated ATPase. The large amount of hemoglobin present in the exocytic vesicle is best explained by trapping of free cytosol and probably reflects a direct interaction of cytosolic components containing hemoglobin with the phospholipid bilayer.