Hollow cylinder protein in the cytoplasm of human erythrocytes

A ‘Hollow Cylinder Protein’ (HCP) similar to the protein originally isolated by Harris from human erythrocyte membranes (Harris, J.R. (1968) Biochim. Biophys. Acta 150, 534–537) is present in the cytosol of erythrocytes at a concentration of more than 15 μg/ml packed erythrocytes. When negatively stained and examined in the electron microscope, cytosol HCP is similar in morphology to the HCP associated with erythrocyte ghost membranes. Cytosol HCP can be purified by isoelectric precipitation at pH 5.2 followed by repeated sucrose gradient centrifugation at alkaline pH. Negatively stained purified cytosol HCP appears as a hollow cylinder with apparent dimensions of 18.0 nm in length by 11.8 nm in diameter and contains a hollow core. Purified cytosol HCP migrates as a single band by non-denaturing polyacrylamide gel electrophoresis. SDS-polyacrylamide gel electrophoresis shows that it is composed of five peptides having apparent molecular weights 21 500, 23 500, 26 000, 27 500 and 29 000. Chymotryptic peptide maps of each of these bands indicate that each is a unique polypeptide chain. These results indicate that erythrocyte cytosol HCP is a macromolecular complex composed of multiple copies of five non-identical subunits arranged as a hollow cylinder.