A conserved phosphoprotein that specifically binds nuclear localization sequences is involved in nuclear import

Area of Science:

• Molecular Biology
• Cell Biology
• Biochemistry

Background:

• Nuclear localization sequences (NLSs) mediate protein import into the nucleus.
• A yeast NLS-binding protein (NBP) has been previously identified.
• The function and conservation of NLS-binding proteins across eukaryotes remain to be fully elucidated.

Purpose of the Study:

• To identify and characterize NLS-binding proteins (NBPs) in different eukaryotic organisms.
• To investigate the role of NBPs in the nuclear import pathway.
• To determine if NBPs function as receptors for NLS-containing proteins.

Main Methods:

• Purification of 70 kD proteins from Drosophila, HeLa cells, and Z. mays.
• Antibody characterization using antibodies against yeast NBP.
• Phosphorylation and phosphatase treatment assays.
• Nuclear import assays in semipermeabilized Drosophila melanogaster cells.
• Inhibition studies using anti-NBP antibodies.

Main Results:

• Conserved 70 kD NBPs were purified from multiple eukaryotic species (Drosophila, HeLa, Z. mays).
• NBPs are present in both cytosol and nuclei and are phosphorylated.
• Phosphatase treatment abolished NLS binding activity.
• NBPs are required for nuclear protein import, mediating the binding of NLS-containing proteins to cytosolic structures and the nuclear envelope.
• Anti-NBP antibodies inhibited the initial binding step of nuclear import.

Conclusions:

• A conserved phosphoprotein functions as a receptor recognizing NLSs.
• This NLS-binding protein (NBP) is crucial for the initial recognition step in nuclear protein import across eukaryotes.
• NBPs play a fundamental role in regulating nuclear transport.