Unique residues at the 3-fold and 4-fold axis of mycobacterial ferritin are involved in oligomer switching.

To identify the crucial residues involved in the self-assembly and function of BfrB, one of the important iron storage proteins of Mycobacterium tuberculosis, we constructed various mutants by employing site-directed mutagenesis. The analysis of mutants led to the identification of "interface hot-spot residues" (R69, L129, and F159) that act as "switch points" for BfrB oligomerization, and our observations show the importance of 4-fold axis residues in assembly formation. Moreover, we demonstrate that single-point mutations Q51A, Q126A, and E135A can enhance the thermal stability of the protein without affecting its assembly. Importantly, a comparative analysis of various mutations revealed that the function of various homologous positions in different ferritins could be at variance; hence, predicting the function of a residue just based on sequence-structure comparisons may not be appropriate. Thus, we report the identification of novel residues in the assembly formation and function of BfrB and show that single-point mutations have a remarkable potential for alteration of multiple properties of ferritins. Besides, "switch residues" or "interface hot spots" identified in this study could also prove to be helpful for the rational design of interfacial inhibitors.