Nucleocytoplasmic Transport

Nucleocytoplasmic transport is central to the functioning of eukaryotic cells and is an integral part of the processes that lead to most human diseases. Over the past fifteen years, we have elucidated the molecular basis for recognition of classical (Cingolani et al., Nature, 1999) and non-classical (Cingolani et al., Mol. Cell, 2002) import substrates by the ubiquitous transport factor importin b (also known as Karyopherin b). In the so-called 'classical' import reaction, the receptor importin b heterodimerizes with the adaptor importin a, that in turn associates directly with proteins bearing a classical Nuclear Localization Signal (NLS) via its helical Arm-core (Fig. 1). The trimeric import complex docks to the Nuclear Pore Complex (NPC) and translocates through the NPC via multiple rounds of interactions of importin b with FG-rich nucleoporins lining the NPC, in a process that requires the GTPase Ran.

Nuclear Import Complex

Figure 1. A structural model of the import complex based on the structure of human importin a and b

The human genome encodes seven isoforms of the adaptor importin a that share high sequence similarity (Pumroy and Cingolani, Biochemical J., 2015) (Fig. 2). All isoforms share a fundamentally conserved architecture that consists of an N-terminal auto-inhibitory importin b binding (IBB)-domain and a C-terminal Arm-core that associates with NLS-cargos. Despite the striking similarity in aminoacid sequence and 3D-structure, importin α isoforms display remarkable substrate specificity in vivo. While all importin a isoforms can import substrates bearing a classical NLS, only certain isoforms bind to dimeric transcription factors like STATs and NF-kB(p50:p65), a phenomenon that we named 'isoform-specialization'.

Importin alpha isoforms

Figure 2. Phylogenetic trees of yeast, Drosophila and human isoforms of importin a. The electrostatic surface charge distribution of importin α isoforms determined crystallographically is shown next to the respective gene.

In my laboratory, we are interested in understanding the mechanisms by which nuclear transport is regulated under physiological conditions and in diseased states. Our research is important to understand how living cells regulate the availability of essential factors bearing nuclear activity (e.g. transcription factors, DNA replication factors, etc). This is emerging as a novel and very powerful way to control gene expression and cellular differentiation. Likewise, misregulation of nuclear transport both in over-proliferating tumor cells and in cells hijacked by pathogenic viruses is emerging as a critically important target for pharmacological intervention.