Major nucleolar proteins shuttle between nucleus and cytoplasm.

Area of Science:

• Molecular Biology
• Cell Biology
• Biochemistry

Background:

• Nucleolin (92 kDa) regulates RNA polymerase I transcription and preribosomal RNA binding.
• B23/No38 (38 kDa) is involved in intranuclear packaging of preribosomal particles.
• Both proteins were previously thought to be exclusively nuclear.

Purpose of the Study:

• To investigate the localization and movement of nucleolin and B23/No38 proteins.
• To determine if these nucleolar proteins shuttle between the nucleus and cytoplasm.
• To explore the functional implications of nucleocytoplasmic shuttling for these proteins.

Main Methods:

• Interspecies heterokaryon analysis to monitor protein equilibration between nuclei.
• Cytoplasmic microinjection of antibodies followed by observation of antigen-mediated nuclear accumulation.
• Immunological detection of protein localization and movement.

Main Results:

• Evidence of constant shuttling for both nucleolin and B23/No38 between nucleus and cytoplasm.
• Demonstration of antigen-mediated nuclear import of cytoplasmically introduced antibodies.
• Unexpected detection of these major nucleolar proteins in the cytoplasm.

Conclusions:

• Nucleolin and B23/No38 proteins actively shuttle between the nucleus and cytoplasm.
• These shuttling proteins likely play a role in nucleocytoplasmic transport of ribosomal components.
• Cytoplasmic exposure of shuttling proteins may enable cytoplasmic regulation of nuclear functions.