Steady-state nuclear actin levels are determined by export competent actin pool

Area of Science:

• Cell Biology
• Molecular Biology
• Biochemistry

Background:

• Actin is increasingly recognized as a key player within the nuclear compartment, involved in critical functions such as transcription and chromatin remodeling.
• Nuclear actin levels are dynamic and linked to cellular states, with decreased levels associated with quiescence and increased levels with differentiation.
• While importin 9 and exportin 6 mediate actin nucleocytoplasmic transport, the mechanisms regulating nuclear actin levels remain poorly understood.

Purpose of the Study:

• To investigate the cellular parameters and transport mechanisms that contribute to the observed cell-to-cell variability in nuclear actin levels.
• To elucidate how actin import and export rates are modulated to maintain distinct nuclear actin concentrations.

Main Methods:

• Quantification of various cellular parameters in cells under normal growth conditions.
• Simultaneous recording of import and export rates of green fluorescent protein (GFP)-tagged actin.
• Correlation analysis between nuclear actin content and actin transport rates.

Main Results:

• Nuclear actin levels exhibit significant cell-to-cell variation under normal conditions.
• The ratio of nuclear to cytoplasmic actin fluorescence negatively correlates with both actin import and export rates.
• Cells with high nuclear actin show reduced actin mobility, indicating sequestration through binding to nuclear complexes, rendering it export-incompetent.

Conclusions:

• High nuclear actin content is actively maintained by a coordinated reduction in both actin import and export.
• The formation of an export-incompetent nuclear actin pool is a key mechanism for ensuring sufficient actin availability for nuclear functions.
• This regulatory mechanism highlights the importance of precise control over nuclear actin dynamics for cellular processes.