The cell-surface-expressed nucleolin is associated with the actin cytoskeleton.

Area of Science:

• Cell Biology
• Molecular Biology
• Biochemistry

Background:

• Nucleolin is a known RNA- and protein-binding protein primarily localized in the nucleolus.
• It is also found intracellularly in the nucleus and cytoplasm, and notably, expressed on the cell surface.
• The precise mechanisms and functions of cell-surface nucleolin are not fully understood.

Purpose of the Study:

• To investigate the mechanism of nucleolin translocation to the cell surface.
• To characterize the functional role of cell-surface nucleolin.
• To determine if cell-surface nucleolin acts as a receptor.

Main Methods:

• Confocal and electron microscopy using anti-nucleolin antibodies.
• Assays involving low temperature, serum-free medium, and glycoprotein transport inhibitors.
• Cell surface cross-linking with a nucleolin-specific monoclonal antibody (mAb D3) at different temperatures.
• Assessment of actin cytoskeleton involvement.

Main Results:

• Cytoplasmic nucleolin is translocated to the cell surface via small vesicles through an active, Golgi-independent pathway.
• Translocation is sensitive to temperature and medium conditions but not to standard glycoprotein transport inhibitors.
• Cell-surface nucleolin clusters upon antibody cross-linking, dependent on an intact actin cytoskeleton.
• Internalization of the antibody-nucleolin complex at 37°C indicates nucleolin mediates ligand import.

Conclusions:

• Nucleolin is actively transported to the cell surface through a novel pathway.
• Cell-surface nucleolin functions as a receptor capable of mediating the import of specific ligands.
• These findings expand the known roles of nucleolin beyond its intracellular functions.