The role of cyclin D2 and p21/waf1 in human T-cell leukemia virus type 1 infected cells.
BACKGROUND
The human T-cell leukemia virus type 1 (HTLV-1) Tax protein indirectly influences transcriptional activation, signal transduction, cell cycle control, and apoptosis. The function of Tax primarily relies on protein-protein interactions. We have previously shown that Tax upregulates the cell cycle checkpoint proteins p21/waf1 and cyclin D2. Here we describe the consequences of upregulating these G1/S checkpoint regulators in HTLV-1 infected cells.


RESULTS
To further decipher any physical and functional interactions between cyclin D2 and p21/waf1, we used a series of biochemical assays from HTLV-1 infected and uninfected cells. Immunoprecipitations from HTLV-1 infected cells showed p21/waf1 in a stable complex with cyclin D2/cdk4. This complex is active as it phosphorylates the Rb protein in kinase assays. Confocal fluorescent microscopy indicated that p21/waf1 and cyclin D2 colocalize in HTLV-1 infected, but not in uninfected cells. Furthermore, in vitro kinase assays using purified proteins demonstrated that the addition of p21/waf1 to cyclin D2/cdk4 increased the kinase activity of cdk4.


CONCLUSIONS
These data suggest that the p21/cyclin D2/cdk4 complex is not an inhibitory complex and that p21/waf1 could potentially function as an assembly factor for the cyclin D2/cdk4 complex in HTLV-1 infected cells. A by-product of this assembly with cyclin D2/cdk4 is the sequestration of p21/waf1 away from the cyclin E/cdk2 complex, allowing this active cyclin-cdk complex to phosphorylate Rb pocket proteins efficiently and push cells through the G1/S checkpoint. These two distinct functional and physical activities of p21/waf1 suggest that RNA tumor viruses manipulate the G1/S checkpoint by deregulating cyclin and cdk complexes.
Kehn
Kylene
K
Department of Biochemistry and Molecular Biology, The George Washington University Medical Center, Washington, DC 20037, USA. bcmkwk@gwumc.edu
Deng
Longwen
L
de la Fuente
Cynthia
C
Strouss
Katharine
K
Wu
Kaili
K
Maddukuri
Anil
A
Baylor
Shanese
S
Rufner
Robyn
R
Pumfery
Anne
A
Bottazzi
Maria Elena
ME
Kashanchi
Fatah
F
D016475:3T3 Cells; D000818:Animals; D017209:Apoptosis; D002453:Cell Cycle; D002460:Cell Line; D002478:Cells, Cultured; D002843:Chromatin / Q000235:genetics; D056742:Cyclin D2; D050759:Cyclin-Dependent Kinase Inhibitor p21 / Q000235:genetics / Q000502:physiology*; D016213:Cyclins / Q000235:genetics / Q000502:physiology*; D005347:Fibroblasts; D016356:Gene Products, tax / Q000502:physiology; D015368:Human T-lymphotropic virus 1 / Q000235:genetics / Q000502:physiology*; D006801:Humans; D051379:Mice; D018613:Microscopy, Confocal; D015533:Transcriptional Activation
C080336:CCND2 protein, human; C493956:CDKN1A protein, human; D002843:Chromatin; D056742:Cyclin D2; D050759:Cyclin-Dependent Kinase Inhibitor p21; D016213:Cyclins; D016356:Gene Products, tax