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cdc2-like kinase is associated with the retinoblastoma protein
M Kitagawa1, S Saitoh, H Ogino
1Tsukuba Research Institute, Banyu Pharmaceuticals Co. Ltd., Japan.
Abstract:
The growth-suppressive activity of the retinoblastoma (RB) protein is suggested to be regulated by phosphorylation. In studies on the kinase that phosphorylates the RB proteins, we have previously found that RB proteins can be phosphorylated by purified cdc2 kinase. In this study, we noted that RB proteins immunoprecipitated from human cell lysates are weakly phosphorylated in the absence of purified cdc2 kinase. Immunoblot analysis showed the presence of p34cdc2 in the immunoprecipitates with anti-RB monoclonal antibody. In addition, the coprecipitated kinase was found to have the same substrate specificity as cdc2 kinase. The associated kinase activity was particularly high in cells arrested in G1/S and S phase by aphidicolin. Furthermore, RB proteins were shown to be phosphorylated in nuclear extracts by some endogenous cdc2-like kinase(s). These results suggest that cdc2-like kinase is the main kinase for phosphorylation of RB proteins in vivo.
Insights
Retinoblastoma (RB) protein phosphorylation, crucial for its growth suppression, is mainly mediated by cdc2-like kinases in human cells. This study identifies endogenous cdc2-like kinase as the primary enzyme responsible for RB protein phosphorylation in vivo.
Area of Science:
- Cell Biology
- Molecular Biology
- Biochemistry
Background:
- The retinoblastoma (RB) protein's growth-suppressive function is modulated by phosphorylation.
- Previous research indicated that purified cdc2 kinase can phosphorylate RB proteins.
Purpose of the Study:
- To investigate the endogenous kinase responsible for retinoblastoma (RB) protein phosphorylation in human cells.
- To determine if cdc2-like kinases are involved in RB protein phosphorylation in vivo.
Main Methods:
- Immunoprecipitation of RB proteins from human cell lysates.
- Immunoblot analysis to detect associated proteins and kinase activity.
- Phosphorylation assays using nuclear extracts and RB proteins.
Main Results:
- RB proteins immunoprecipitated from cell lysates showed weak phosphorylation without added cdc2 kinase.
- p34cdc2 was detected in RB immunoprecipitates, with associated kinase activity similar to cdc2 kinase.
- Associated kinase activity was elevated in G1/S and S phase-arrested cells.
- Endogenous cdc2-like kinases phosphorylated RB proteins in nuclear extracts.
Conclusions:
- cdc2-like kinase is the primary kinase responsible for retinoblastoma (RB) protein phosphorylation in vivo.
- This finding clarifies the regulation of RB protein activity by specific kinases within the cellular environment.