Importin α2 association with chromatin: Direct DNA binding via a novel DNA binding domain

Abstract

Nuclear transport of proteins is important for facilitating appropriate nuclear functions. The proteins of the importin family play key roles in nuclear transport as transport receptors for a huge number of nuclear proteins. Additionally, these proteins possess other functions, including chromatin association and gene regulation. However, these non-transport functions of importin are not yet fully understood, especially their molecular-level mechanisms for functioning with chromatin and their consequences. Here, we report the novel molecular characteristics of importin involving binding to diverse sequences in chromatin. We newly identified and characterized a DNA-binding domain--the Nucleic Acid Associating Trolley pole domain (NAAT domain)--in the N-terminal region of importin within the conventional importin {beta} binding (IBB) domain, which was shown to be necessary for nuclear transport of cargo proteins. We propose a stroll and locate model to explain the association of importin with chromatin. This is the first study to delineate the interaction between importin and chromatin DNA via the NAAT domain, indicating the bifunctionality of the importin N-terminal region for nuclear transport and chromatin association.