Conformational heterogeneity of karyopherin beta2 is segmental

Area of Science:

• Structural Biology
• Molecular Cell Biology
• Biochemistry

Background:

• Karyopherin beta2 (Kap beta2), also known as transportin, is crucial for nuclear import of RNA-binding proteins.
• Kap beta2 facilitates transport by binding substrates in the cytoplasm and releasing them in the nucleus via RanGTP interaction.
• Understanding Kap beta2's structure is key to elucidating nuclear transport regulation.

Purpose of the Study:

• To determine the crystal structure of unliganded Karyopherin beta2 (Kap beta2).
• To understand the structural basis of Kap beta2's conformational flexibility upon ligand binding.
• To compare the structural architecture of Kap beta2 with other import karyopherins like Importin beta.

Main Methods:

• X-ray crystallography was used to obtain the 3.0 Å crystal structure of unliganded Kap beta2.
• Analysis of the crystal structure focused on the arrangement of HEAT repeats and identification of distinct functional segments.
• Comparative structural analysis with existing NLS and Ran complexes of Kap beta2 and Importin beta.

Main Results:

• The crystal structure reveals Kap beta2 comprises a superhelix of 20 HEAT repeats, divided into three major segments.
• Two segments (HEAT repeats 9-13 and 14-18) form the substrate binding site and exhibit rigid-body rotation around a flexible hinge.
• The third segment (HEAT repeats 1-8) forms the Ran binding site and shows conformational flexibility along its length.

Conclusions:

• The unliganded Kap beta2 structure reveals a segmented architecture crucial for its function in nuclear transport.
• The distinct rigid and flexible segments likely accommodate conformational changes necessary for substrate and RanGTP binding.
• This segmental organization may be a conserved feature among import karyopherins, including Importin beta.