Toxoplasma gondii importin α shows weak auto-inhibition

Importin is a nuclear transporter that binds to nuclear localization signals (NLSs), consisting of 7-20 positively charged amino acids found within cargo proteins. In addition to cargo binding, intramolecular interactions also occur within the importin protein due to binding between the importin {beta}-binding (IBB) domain and the NLS-binding sites, a phenomenon called auto-inhibition. The interactions causing auto-inhibition are driven by a stretch of basic residues, similar to an NLS, in the IBB domain. Consistent with this, importin proteins that do not have some of these basic residues lack auto-inhibition; a naturally occurring example of such a protein is found in the apicomplexan parasite Plasmodium falciparum. In this report, we show that importin from another apicomplexan parasite, Toxoplasma gondii, harbors basic residues (KKR) in the IBB domain and exhibits auto-inhibition. This protein has a long, unstructured hinge motif (between the IBB domain and the NLS-binding sites) that does not contribute to auto-inhibition. However, the IBB domain may have a higher propensity to form an -helical structure, positioning the wild-type KKR motif in an orientation that results in weaker interactions with the NLS-binding site than a KRR mutant. We conclude that the importin protein from T. gondii shows auto-inhibition, exhibiting a different phenotype from that of P. falciparum importin . However, our data indicate that T. gondii importin may have a low strength of auto-inhibition. We hypothesize that low levels of auto-inhibition may confer an advantage to these important human pathogens.