Substrates of CDKs

Due to the problems in identification of cellular substrates of protein kinases, as described in Chapter 7, it has been a difficult and lengthy process to determine the functionally relevant substrates. Pig. 13.11 gives an overview of the ceU-cycle-specific activation of CDKs and some important substrates. Comparatively sparse information is available on the Gj and S phase substrates of the CDKs. In contrast, many proteins have been described that undergo specific phosphorylation in G2/M phase. The sequence (K/R)-S/T-P-X-K (X any amino acid) has been identified as a consensus sequence for phosphorylation by CDKs. 

Important substrates in Gi/S phase are the transcription factor E2P, the tumor suppressor protein pRb and pRb-related proteins. E2P represents a family of heterodimeric transcription factors with activity strictly regulated in the cell cycle. The members of the E2P family regulate transcription of genes with products that are required for the 

G]/S transition and for progess through S phase. Tlie activity of E2F may be regulated by CDK-mediated phosphorylation and by association with pRb. The tumor suppressor protein pRb is itself a substrate for phosphorylation by the CDK-cychn D1 complex. Both proteins are described in detail in Chapter 14. 

In M phase, new phosphorylation of many proteins is observed that starts, in particular, from the CDC2-cyclin B complex. The phosphorylation mostly affects proteins involved in the reorganization of the cytoskeleton, the nuclear membrane and the formation of the spindle apparatus. As a consequence of phosphorylation events, inhibition of vesicular transport and general inhibition of transcription occur. 

Examples of proteins that are specifically phosphorylated during the cell cycle are the lamins. Hyperphosphorylation of the lamins leads to disintegration of the nuclear lamina. Myosin in actin-myosin filaments is also specifically phosphorylated during mitosis. Other M-phase-specific phosphorylations occur at transcription factor TFIIIB, leading to inhibition of transcription by RNA polymerase III. Phosphorylation of TAP proteins (see 1.4.2.3) is also involved in general inhibition of banscription. 

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Binding of protein substrates to CDK-cyclin complexes is not restricted to CDKs. Rather, cyclins contain structural elements that mediate interactions with CKIs and with CDK substrates. Thus, the role of cyclins has to be extended to selection of binding substrates of CDKs. 

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