The structure of partially expanded erythrocyte spectrin

Abstract

In vivo the red cell suffers elastic deformation during its turbulent passage through the vasculature. Although the physical process responsible for the cell’s elasticity is still unknown, it is generally regarded as being a property of the erythrocyte skeleton. The skeleton, a two-dimensional network located on the cytoplasmic side of the membrane is composed of dodecamers of actin interconnected by spectrin molecules. Understanding the elastic properties of the skeleton in terms of the molecular properties of its component proteins is a major unsolved problem in red cell biology. Our work has focused on spectrin since it alone, of the skeleton proteins, appears to be able to undergo the reversible changes in structure necessary to account for the elastic properties of the red cell.Spectrin is highly alpha-helical and highly charged. Consequently the molecule can reversibly expand or condense in response to changes in ionic strength. Spectrin forms tetramers which are 2000 Å long when fully extended. The conformation actually assumed by spectrin in the cell is not known; however, calculations based on the number of molecules per unit area of membrane indicate that in the erythrocyte the average end to end distance of the molecule is only 700 Å.