Ultrastructure of Two Mutants in the H-Subunit of Recombinant Human Ferritin: Ordered Arrays Indicate Altered Surface Properties
Two major types of polypeptide subunits, H and L, coassemble (n = 24) as heteropolymers to form molecules of the iron-storage protein, ferritin [1]. The significance of the H-type subunits for molecular structure and function remains unclear. The present study has examined the ultrastructure of recombinant human ferritins which are homopolymers of wild-type or mutant H-subunits [2]. Both mutations change the aminoacid sequence along the outer surface of the molecule; 9Cd has a single substitution of glycine for the lysine at position 86, and Ml has a duplication of 18 aminoacids [2].Recombinant ferritins were overexpressed in E. coli. and purified as described by Levi et al [2]. Samples (0.1-0.5 mg/ml) were deposited onto a thin carbon film stripped from freshly cleaved mica and carried on 400 mesh grids; these specimen carriers had been made hydrophilic by glow discharge treatment. After negative staining with 2% sodium silicotungstate (pH 7.0), the specimens were dried and examined at lOOkV with a JEOL 100CX electron microscope.