Mutating one or more of the following residues in subdomain I results in a failure of cyclin binding to Cdk2: Glu8, Lys9, Glu12. This suggests that cyclin may interact with or recognize these sites.
The glycine-rich loop of the ATP binding site, which has the consensus sequence Gly-x-Gly-x-x-Gly-Val is located in subdomain I. The glycine-rich loop motif in Cdk2 starts at Gly11 and can be found between beta-1 and beta-2, in the L2 region. The actual sequence is Gly-Glu-Gly-Thr-Tyr-Gly-Val. This glycine-rich loop serves as a phosphate anchor, forming bonds to the phosphates of the ATP. The glycine-rich loop is positioned differently in Cdk2 than it is in cAPK. This variation may be what causes the difference in the orientation of bound ATP between the two types of kinases. In Cdk2, the loop is closer to the ATP, which may be preventing the active beta-phosphate conformation seen in cAPK bound with ATP. Thr14 and Tyr15 of the glycine-rich loop are the molecule's inhibitory sites that act to regulate Cdk2 activity. A phosphate bound to the Thr14 hydroxyl group would prevent another ATP from entering the pocket without disrupting the conformation of either molecule. Therefore, it is not sterically possible to fit the ATP needed to phosphorylate Threonine 160 of the T-loop, located in subdomain VIII. Phosphorylation at either Thr14 or Thr15 may also interfere with protein substrate binding or change the conformation of the glycine-rich loop, preventing ATP from binding in the right orientation, thus inhibiting kinase activity.