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Mol. Cell. Biol., Dec 1997, 6815-6821, Vol 17, No. 12
RW Deed, E Hara, GT Atherton, G Peters and JD Norton
The functions of basic helix-loop-helix (bHLH) transcription factors in
activating differentiation-linked gene expression and in inducing G1 cell
cycle arrest are negatively regulated by members of the Id family of HLH
proteins. These bHLH antagonists are induced during a mitogenic signalling
response, and they function by sequestering their bHLH targets in inactive
heterodimers that are unable to bind to specific gene regulatory (E box)
sequences. Recently, cyclin E-Cdk2- and cyclin A-Cdk2-dependent
phosphorylation of a single conserved serine residue (Ser5) in Id2 has been
shown to occur during late G1-to-S phase transition of the cell cycle, and
this neutralizes the function of Id2 in abrogating E-box-dependent bHLH
homo- or heterodimer complex formation in vitro (E. Hara, M. Hall, and G.
Peters, EMBO J. 16:332- 342, 1997). We now show that an analogous
cell-cycle-regulated phosphorylation of Id3 alters the specificity of Id3
for abrogating both E-box-dependent bHLH homo- or heterodimer complex
formation in vitro and E-box-dependent reporter gene function in vivo.
Furthermore, compared with wild-type Id3, an Id3 Asp5 mutant (mimicking
phosphorylation) is unable to promote cell cycle S phase entry in
transfected fibroblasts, whereas an Id3 Ala5 mutant (ablating
phosphorylation) displays an activity significantly greater than that of
wild-type Id3 protein. Cdk2-dependent phosphorylation therefore provides a
switch during late G1-to-S phase that both nullifies an early G1 cell cycle
regulatory function of Id3 and modulates its target bHLH specificity. These
data also demonstrate that the ability of Id3 to promote cell cycle S phase
entry is not simply a function of its ability to modulate bHLH
heterodimer-dependent gene expression and establish a biologically
important mechanism through which Cdk2 and Id- bHLH functions are
integrated in the coordination of cell proliferation and differentiation.
Copyright © 1997, American Society for Microbiology
Regulation of Id3 cell cycle function by Cdk-2-dependent phosphorylation
CRC Department of Gene Regulation, Paterson Institute for Cancer Research, Christie Hospital NHS Trust, Manchester, United Kingdom.
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