GENETIC SELECTION OF PEPTIDE APTAMERS THAT RECOGNIZE AND INHIBIT CYCLIN-DEPENDENT-KINASE-2

A NETWORK of interacting proteins controls the activity of cyclin-depe ndent kinase 2 (Cdk2) (refs 1, 2) and governs the entry of higher euka ryotic cells into S phase. Analysis of this and other genetic regulato ry networks would be facilitated by intracellular reagents that recogn ize specific targets and inhibit specific network connections. We repo rt here the expression of a combinatorial library of constrained 20-re sidue peptides displayed by the active-site loop of Escherichia coli t hioredoxin, and the use of a two-hybrid system to select those that bi nd human Cdk2. These peptide aptamers were designed to mimic the recog nition function of the complementarity-determining regions of immunogl obulins. The aptamers recognized different epitopes on the Cdk2 surfac e with equilibrium dissociation constant in the nanomolar range; those tested inhibited Cdk2 activity. Our results show that peptide aptamer s bear some analogies with monoclonal antibodies, with the advantages that they are isolated together with their coding genes, that their sm all sizes should allow their structures to be solved, and that they ar e designated to function inside cells.