A non-canonical mechanism for Crm1-export cargo complex assembly.

Area of Science:

• Molecular Biology
• Cell Biology
• Biochemistry

Background:

• The transport receptor Crm1 (chromosome region maintenance 1) mediates nuclear export of cargos with leucine-rich nuclear export signals (NESs) via RanGTP complex formation.
• Efficient cargo release in the cytoplasm requires NESs to have low affinity for Crm1, posing a challenge for nuclear complex assembly.

Purpose of the Study:

• To elucidate the mechanisms underlying the assembly of Crm1-export complexes in the nucleus, particularly how low-affinity interactions are overcome.
• To identify novel factors involved in Crm1-mediated nuclear export.

Main Methods:

• In vitro binding assays to characterize protein-protein interactions.
• Biochemical analyses to determine complex formation and function.
• Genetic manipulation (mutagenesis) to assess the impact on protein function and cellular processes.

Main Results:

• A new RanGTP-binding protein, Slx9, was identified as a crucial factor in Crm1-mediated export.
• Slx9 acts as a scaffold, binding both Rio2 (an adaptor for 40S pre-ribosomes) and RanGTP.
• This Slx9-Rio2-RanGTP complex directly recruits Crm1, enabling a non-canonical stepwise assembly of the export complex.
• A mutation in Slx9 that disrupts Crm1-export complex assembly inhibits 40S pre-ribosome export.

Conclusions:

• Slx9 functions as a molecular scaffold, optimizing the presentation of RanGTP and the NES-containing adaptor Rio2 to Crm1.
• This mechanism facilitates the efficient assembly of Crm1-export complexes, triggering 40S pre-ribosome export.
• The Slx9-mediated pathway offers a solution to the paradox of weak binding interactions driving rapid Crm1-dependent nuclear export.