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Analysis of Cell Cycle Position in Mammalian Cells
12:19

Analysis of Cell Cycle Position in Mammalian Cells

Published on: January 21, 2012

The retinoblastoma gene product is bound in the nucleus in early G1 phase

T Stokke1, B K Erikstein, L Smedshammer

  • 1Department of Biophysics, Norwegian Radium Hospital, Montebello, Oslo.

Experimental Cell Research
|January 1, 1993
PubMed

Insights

The retinoblastoma susceptibility gene product (pRB) is underphosphorylated and nuclear-bound only in early G1 phase. Its nuclear binding duration, not phosphorylation timing, may control cell cycle G1 phase length.

Area of Science:

  • Cell Biology
  • Molecular Biology
  • Cancer Research

Background:

  • The retinoblastoma susceptibility gene product (pRB) is a key regulator of cell cycle progression.
  • pRB's growth-regulatory functions are primarily associated with the G1 phase.
  • Previous assumptions suggested pRB is underphosphorylated throughout G1.

Purpose of the Study:

  • To investigate the precise localization and phosphorylation state of pRB during the G1 phase.
  • To determine the role of pRB's nuclear binding and phosphorylation in cell cycle regulation.
  • To clarify pRB's involvement in the initiation of DNA replication.

Main Methods:

  • Cell cycle analysis
  • Immunofluorescence microscopy
  • Biochemical assays to determine pRB phosphorylation and binding status

Main Results:

  • pRB is underphosphorylated and tightly bound in the nucleus exclusively during early G1 phase.
  • All quiescent (G0) cells examined contained bound, underphosphorylated pRB.
  • The duration of G1 phase and the overall cell cycle appears to correlate with the period of nuclear pRB binding.
  • A consistent 6.5-hour time lag was observed between pRB phosphorylation/release and S phase entry, irrespective of G1 transit time.

Conclusions:

  • pRB's underphosphorylated, nuclear-bound state is restricted to early G1.
  • The duration of pRB nuclear binding, rather than its phosphorylation status, may dictate G1 phase length.
  • Data indicate pRB is likely not directly involved in initiating DNA replication.

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