Subdomain III is made up of residues 44-58 which form the large alpha-1 helix, the only alpha helix of the small lobe. This includes the PSTAIRE sequence, a conserved region of the cyclin dependent kinases. Mutations in this region have shown to abolish cyclin binding. When cyclin A binds to Cdk2, the conformation of the PSTAIRE region changes, ending up near the C terminus of cyclin's alpha-3 helix on one side and the alpha-5 helix, which runs parallel to the PSTAIRE helix on the other side. The middle of the PSTAIRE helix that is intercepted by the alpha-3 helix binds to it by hydrophobic interactions, whereas the C and N-terminal regions are bind cyclin through hydrogen bonds. A hydrophobic pocket within cyclin A made from side chains of Leu263, Phe267, Leu299, Leu306, and Lys266 fit tightly around Ile49. Val44 along with Glu42 of subdomain II accept two hydrogen bonds from the Lys266 side chain of cyclin A. The backbone amide of Val44 donates a hydrogen bond to the Glu295 carboxylate of cyclin A. Lys266 and Glu295 side chains are also linked by hydrogen bonds to Cdk2. Gly43, an important conserved residue due to the unique backbone conformation that it can adopt(phi=112 degrees, psi=-150 degrees), allows this hydrogen-bond network to occur. The E in the PSTAIRE sequence corresponds to Glu51, which is homologous to Glu91 in cAPK. However, the stabilizing interaction between the lysine and glutamate residues that is seen in cAPK between Lys79 and Glu91 does not occur in Cdk2, due to the presence of the alpha L12 helix in the C terminal lobe, which pushes the alpha 1 helix further away from the Lys33. The alpha-1 helix of Cdk2 therefore has an orientation and position that differs from that of the corresponding Helix C in cAPK. However, once cyclin A binds Cdk2, the conformation changes so that the two residues can interact. This partially activates the complex.