Structure, assembly, conformation, and immunological properties of the two subunit classes of ferritin

Abstract

The 2 subunit types of human liver ferritin were purified to homogeneity. Both subunits reassembled in a well-defined manner and formed spherical particles that resembled natural apoferritin in EM. Affinity chromatography methods were employed to obtain preparations of antibodies that interacted exclusively either with the H or with the L polypeptides, demonstrating that distinct immunological properties may be ascribed to each subunit of ferritin. The amino acid compositions of the subunits were similar but the larger H subunit had fewer leucine, phenyl-alanine and arginine residues. It is therefore improbable that H subunits undergo proteolytic processing and are precursors for L subunits. Circular dichroism data indicated that homopolymers assembled from L-type subunits had substantially more ordered secondary structures and greater .alpha.-helical contents than their H counterparts. Small differences in the environment of tryptophan residues were evident from fluorescence spectra of each homopolymer. In isoelectric focusing experiments reassembled H or L homopolymers migrated as families of proteins within discrete pI [isoelectric point] ranges which are probably representative of subpopulations of each subunit type. The H homopolymer focused at lower pI''s than the L component. These data substantiate the contention that both subunits are authentic polypeptide moieties of ferritin with some common structural features but the results also underscore prominent dissimilarities in their properties.