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The COOH terminus of p18INK4C distinguishes function from p16INK4A
1Department of Molecular Physiology, University of Vermont, Burlington, VT 05405, USA.
Abstract:
The INK4 family of proteins consists of four members which can block progression from the G(1)-to-S phase of the cell cycle by inhibiting the activity of cyclin dependent kinases (cdks) 4 and 6. Although the gene encoding p16(INK4a) is commonly inactivated in human tumors, p18(INK4c) is rarely altered. We show here that overexpression of p18(INK4c) does not block cell cycle progression in a T-cell acute lymphocytic leukemia cell line (CEM) sensitive to p16(INK4a)-mediated G(1) arrest. A chimera consisting of the kinase-binding region of p16(INK4a) fused to the COOH terminus of p18(INK4c) is active in all known biochemical assays for INK4 function, but it does not arrest CEM cells. These data imply a novel level of p18(INK4c) regulation mediated through the COOH terminus and suggest that functional differences might underlie the distinct mutational profiles observed for p16(INK4a) and p18(INK4c) in tumors.
Insights
The INK4 protein family regulates cell cycle progression. Unlike p16(INK4a), p18(INK4c) shows distinct tumor mutation profiles, suggesting novel regulatory mechanisms.
Area of Science:
- Molecular Biology
- Cell Cycle Regulation
- Oncology
Background:
- The INK4 protein family, including p16(INK4a) and p18(INK4c), inhibits cyclin-dependent kinases (cdks) 4 and 6.
- INK4 proteins block cell cycle progression from G(1) to S phase.
- p16(INK4a) is frequently inactivated in human tumors, whereas p18(INK4c) alterations are rare.
Purpose of the Study:
- To investigate the functional differences between p16(INK4a) and p18(INK4c).
- To explore potential novel regulatory mechanisms of p18(INK4c).
- To understand the distinct mutational profiles of p16(INK4a) and p18(INK4c) in tumors.
Main Methods:
- Overexpression of p18(INK4c) in a T-cell acute lymphocytic leukemia cell line (CEM).
- Assessment of G(1) arrest in response to p18(INK4c) overexpression.
- Construction and functional analysis of a p16(INK4a)/p18(INK4c) chimera.
Main Results:
- p18(INK4c) overexpression did not arrest CEM cells, which are sensitive to p16(INK4a).
- A chimera with the p16(INK4a) kinase-binding region and p18(INK4c) COOH terminus retained INK4 biochemical activity.
- The chimera failed to arrest CEM cells, indicating regulation via the COOH terminus.
Conclusions:
- A novel regulatory mechanism for p18(INK4c) likely involves its COOH terminus.
- Functional differences between p16(INK4a) and p18(INK4c) may explain their distinct tumor mutation patterns.
- Further research into p18(INK4c) regulation is warranted.