The AgNOR proteins: qualitative and quantitative changes during the cell cycle.

Area of Science:

• Cell Biology
• Molecular Biology
• Biochemistry

Background:

• AgNOR proteins are argyrophilic nucleolar proteins that increase in proliferating cells and decrease in non-proliferating cells.
• Some AgNOR proteins remain associated with nucleolar organizer regions (NORs) during mitosis.
• In situ quantification of AgNOR proteins relies on silver staining, morphometry, and image analysis.

Purpose of the Study:

• To understand the relationship between the cell cycle and AgNOR protein quantity.
• To determine specific cell cycle phases where AgNOR protein expression varies.
• To identify the most variable AgNOR proteins during each cell cycle phase.

Main Methods:

• Developed a protocol to detect and quantify AgNOR proteins in electrophoresed protein samples.
• Utilized nitrocellulose membranes for protein transfer.
• Analyzed nucleolar, nuclear, whole interphasic cell, and chromosome-associated protein extracts.

Main Results:

• Identified nucleolin and protein B23 as major AgNOR proteins in the nucleolus during interphase.
• Identified RNA polymerase I subunits and UBF as AgNOR proteins associated with NORs during mitosis.
• Observed that nucleolin and protein B23 levels increase with the cell cycle, not solely due to rDNA transcription stimulation.
• Quantified rapid nucleolin increase in S phase and progressive protein B23 increase peaking in G2 phase.

Conclusions:

• AgNOR protein quantity correlates with cell cycle phases (low in G1, high in S-G2).
• AgNOR protein levels can serve as a proliferation marker.
• AgNOR protein quantification provides insights into the relative proportion and timing of cell cycle phases.