Cell Cycle Regulation
Ligand occupancy of plasma membrane-bound receptors
brings about receptor activation, commonly through
phosphorylation of tyrosine residues, triggering downstream
signal transduction pathways that produce phosphorylated
molecules to act as transcription factors
modulating gene expression (Figure 1). Mutational activation
of any of the component molecules in these cascades
can lead to constitutive signalling in the absence of binding
ligand, and so contribute to tumour development. The
eukaryotic cell cycle is regulated by periodic activation of
different cyclin-dependent kinases (Cdks), heterodimers of
a protein kinase catalytic subunit, the Cdk, and a cyclinactivating
subunit. Different Cdk–cyclin complexes are
required to catalyse the phosphorylation of proteins that