Nuclear import and the evolution of a multifunctional RNA-binding protein

Area of Science:

• Molecular Biology
• Cell Biology
• Protein Transport

Background:

• La (SS-B) is a protein involved in RNA binding and may function as an RNA chaperone.
• Nuclear import pathways regulate protein localization and cellular function.
• Understanding La protein import is crucial for comprehending its role in ribonucleoprotein complexes.

Purpose of the Study:

• To investigate the nuclear import mechanism of the Saccharomyces cerevisiae La protein, Lhp1p.
• To identify the specific domain of Lhp1p responsible for nuclear import mediated by Kap108p.
• To compare the nuclear import pathways of yeast and human La proteins.

Main Methods:

• Characterization of the Lhp1p nuclear import pathway in S. cerevisiae.
• Identification of the Kap108p-interacting domain of Lhp1p.
• Reconstitution of human La nuclear import in vitro using permeabilized HeLa cells and recombinant human factors.

Main Results:

• A 113-amino acid domain of Lhp1p was identified as being imported by Kap108p.
• This domain differs from the nuclear localization signal of human La.
• Nuclear import of S. pombe, Drosophila, and human La proteins in S. cerevisiae is independent of Kap108p.
• Human La nuclear import can be reconstituted using specific human karyopherins (karyopherin alpha2, karyopherin beta1, Ran, and p10).

Conclusions:

• Yeast and human La proteins utilize distinct sequence motifs and karyopherins for nuclear import.
• These findings suggest divergent evolution of La protein import pathways.
• The study highlights the intermingling of nuclear import mechanisms and protein evolution.