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High molecular weight protein phosphatase type 1 dephosphorylates the retinoblastoma protein
D A Nelson1, N A Krucher, J W Ludlow
1Department of Biochemistry and Biophysics, University of Rochester School of Medicine and Dentistry, Rochester, New York 14642, USA.
Abstract:
pRb controls cell proliferation by restricting inappropriate entry of cells into the cell division cycle. As dephosphorylation of pRb during mitotic exit activates its growth suppressive function, identification of the protein phosphatase that dephosphorylates pRb, and characterization of the mechanism of its regulation, are essential to elucidating the mechanisms of cell growth control. By fractionating mitotic CV-1P cell extracts, we identify the protein phosphatase which dephosphorylates pRb as a type 1 serine/threonine phosphoprotein phosphatase (PP1). Molecular sizing analyses indicate that the catalytic enzyme (PP1c) is present in a high molecular weight complex, with a predicted molecular mass of 166 kDa. PP1-interacting proteins in the mitotic cell extracts are identified. Two PP1-interacting proteins (41 and 110 kDa) are shown to form distinct complexes with PP1c from fractions of separated mitotic cell extracts containing phosphorylase phosphatase activity. However, only the 110-kDa PP1-interacting protein is present in fractions containing pRb-directed phosphatase activity, identifying this protein as a putative activator of PP1 function toward pRb during mitosis.
Insights
Researchers identified a specific protein phosphatase, type 1 (PP1), that dephosphorylates pRb, a key regulator of cell proliferation. This finding is crucial for understanding cell growth control mechanisms.
Area of Science:
- Cell Biology
- Molecular Biology
- Biochemistry
Background:
- The retinoblastoma protein (pRb) regulates cell proliferation by controlling cell cycle entry.
- Dephosphorylation of pRb during mitotic exit is essential for its growth-suppressive function.
- Understanding the protein phosphatase responsible for pRb dephosphorylation is key to cell growth control.
Purpose of the Study:
- To identify the specific protein phosphatase that dephosphorylates pRb.
- To characterize the regulatory mechanisms of this phosphatase.
- To elucidate the role of PP1-interacting proteins in pRb dephosphorylation.
Main Methods:
- Fractionation of mitotic cell extracts.
- Molecular sizing analyses.
- Identification of PP1-interacting proteins.
Main Results:
- Type 1 serine/threonine phosphoprotein phosphatase (PP1) was identified as the enzyme that dephosphorylates pRb.
- The catalytic subunit of PP1 (PP1c) exists in a high molecular weight complex (166 kDa).
- A 110-kDa PP1-interacting protein was found to be specifically associated with pRb-directed phosphatase activity, suggesting it activates PP1 towards pRb.
Conclusions:
- PP1 is the primary phosphatase responsible for pRb dephosphorylation.
- A 110-kDa protein acts as a putative activator of PP1, specifically targeting pRb during mitosis.
- This discovery provides critical insights into the regulation of cell proliferation and growth control.