Physicochemical and Functional Comparison ofXenopus laevisNucleoplasmin Obtained from Oocytes and from Overexpression in Bacteria

We compare the physicochemical and functional characteristics of nucleoplasmin obtained fromXenopus laevisoocytes and by bacterial overexpression of a plasmid containing the nucleoplasmin gene. The comparison shows that, while the secondary structure of the protein is not affected by the method used to obtain this protein, the bacterial expressed form exhibits a marked tendency to form large aggregates and an impaired ability to displace protamines from sperm nuclei. These results add a word of caution to the indiscriminate use, in functional or structural (crystallographic) studies, of bacterially overproduced proteins that have been end-terminally tagged with polyhistidine.