Nucleo cytoplasmic trafficking
b. Importin-ß superfamily
Besides passive diffusion three transport pathways have been described for transport of cargo in and out of the nucleus. One class mediates mRNA transport out of the nucleus and uses ATP as energy source (Ashkenazy-Titelman et al, 2020). The second class is a protein solely required for the re-import of the small GTPase Ran which will be discussed under 1.c (Directionality of Transport: The RanGTP-ase System). The third class and majority of nuclear transport receptors belong to the Importin-ß superfamily or ß-karyopherins, named after the first receptor identified. They are responsible for the import and export of proteins or protein complexes, tRNAs as well as some mRNAs. To specify the direction of transport karyopherins have been separated in importins and exportins, although transport receptors have been described that perform translocation of cargo in both ways.
All members of the Importin-ß superfamily share a common structural feature, termed HEAT repeats, first identified in (Huntingtin, elongation factor 3 (EF3), protein phosphatase 2A (PP2A), and the yeast PI3-kinase TOR1(Target of Rapamycin 1). A HEAT repeat consists of two antiparallel alpha-helices separated by a short loop (Fig. 1A.). The N-terminal helix is denoted A-helix and is generally located at the outside of the protein, whereas the second helix, the B-helix is directed to the inner surface of the protein. The stacking arrangement of multiple HEAT repeats leads to a superhelical overall structure as shown for importin-ß, which is composed of 19 HEAT-repeats (Fig.1B.).
Fig. 1. Structural arrangement of the Importin-ß superfamily.
(A) HEAT-repeat structure. (B) Overall Arrangement of HEAT repeats in Importin-ß.
This superhelical arrangement allows for a high dergree of flexibility, enabling the interaction with a wide range of divergent proteins (Fig.3). The shape of the superhelix depicted in green is caused upon binding to an N-terminal alpha helical structure or a protein binding along the C-terminal B-helices within Importin-ß along the central axis. The C-terminal core domain of the bound protein is most likely located on the top of Importin-ß. In the other form of Importin-ß depicted in purple, the cargo is bound to the B-helices in the central region with the main body of the bound molecule presumably lying on the left. The two conformations of Importin-ß differ in respect to overall hight and diameter.
Fig.2. Flexibility of Importin-ß.
The mode of interaction with the cargo causes different conformations of Importin-ß, which show differences in length and diameter.
Besides a high degree of structural similarity, all members of the Importin-ß superfamily share the following binding properties:
The subsequent table lists all the Importin-ß family proteins that have been identified in human and yeast and lists their function.