Novel avidin-like protein from a root nodule symbiotic bacterium, Bradyrhizobium japonicum.

Bradyrhizobium japonicum is an important nitrogenfixing symbiotic bacterium, which can form root nodules on soybeans. These bacteria have a gene encoding a putative avidin- and streptavidin-like protein, which bears an amino acid sequence identity of only about 30% over the core regions with both of them. We produced this protein in Escherichia coli both as the full-length wild type and as a C-terminally truncated core form and showed that it is indeed a high affinity biotin-binding protein that resembles (strept)avidin structurally and functionally. Because of the considerable dissimilarity in the amino acid sequence, however, it is immunologically very different, and polyclonal rabbit and human antibodies toward (strept)avidin did not show significant cross-reactivity with it. Therefore this new avidin, named bradavidin, facilitates medical treatments such as targeted drug delivery, gene therapy, and imaging by offering an alternative tool for use if (strept)avidin cannot be used, because of a deleterious patient immune response for example. In addition to its medical value, bradavidin can be used both in other applications of avidin-biotin technology and as a source of new ideas when creating engineered (strept)avidin forms by changing or combining the desired parts, interface patterns, or specific residues within the avidin protein family. Moreover, the unexpected discovery of bradavidin indicates that the group of new and undiscovered bacterial avidin-like proteins may be both more diverse and more common than hitherto thought.