Streptavidin is one of the most important hubs for molecular biology, either multimerizing biomolecules, bridging one molecule to another, or anchoring to a biotinylated surface/nanoparticle. Streptavidin has the advantage of rapid ultra-stable binding to biotin. However, the ability of streptavidin to bind four biotinylated molecules in a heterogeneous manner is often limiting. Here, we present an efficient approach to isolate streptavidin tetramers with two biotin-binding sites in a precise arrangement, cis or trans. We genetically modified specific subunits with negatively charged tags, refolded a mixture of monomers, and used ion-exchange chromatography to resolve tetramers according to the number and orientation of tags. We solved the crystal structures of cis-divalent streptavidin to 1.4 Å resolution and trans-divalent streptavidin to 1.6 Å resolution, validating the isolation strategy and explaining the behavior of the Dead streptavidin variant. cis- and trans-divalent streptavidins retained tetravalent streptavidin's high thermostability and low off-rate. These defined divalent streptavidins enabled us to uncover how streptavidin binding depends on the nature of the biotin ligand. Biotinylated DNA showed strong negative cooperativity of binding to cis-divalent but not trans-divalent streptavidin. A small biotinylated protein bound readily to cis and trans binding sites. We also solved the structure of trans-divalent streptavidin bound to biotin-4-fluorescein, showing how one ligand obstructs binding to an adjacent biotin-binding site. Using a hexaglutamate tag proved a more powerful way to isolate monovalent streptavidin, for ultra-stable labeling without undesired clustering. These forms of streptavidin allow this key hub to be used with a new level of precision, for homogeneous molecular assembly.

Phospho-CDK2/cyclin A, a kinase that is active in cell cycle S phase, contains an RXL substrate recognition site that is over 40 Å from the catalytic site. The role of this recruitment site, which enhances substrate affinity and catalytic efficiency, has been investigated using peptides derived from the natural substrates, namely CDC6 and p107, and a bispeptide inhibitor in which the ␥-phosphate of ATP is covalently attached by a linker to the CDC6 substrate peptide. X-ray studies with a 30-residue CDC6 peptide in complex with pCDK2/cyclin A showed binding of a dodecamer peptide at the recruitment site and a heptapeptide at the catalytic site, but no density for the linking 11 residues. Kinetic studies established that the CDC6 peptide had an 18-fold lower K m compared with heptapeptide substrate and that this effect required the recruitment peptide to be covalently linked to the substrate peptide. X-ray studies with the CDC6 bispeptide showed binding of the dodecamer at the recruitment site and the modified ATP in two alternative conformations at the catalytic site. The CDC6 bispeptide was a potent inhibitor competitive with both ATP and peptide substrate of pCDK2/ cyclin A activity against a heptapeptide substrate (K i ‫؍‬ 0.83 nM) but less effective against RXL-containing substrates. We discuss how localization at the recruitment site (K D 0.4 M) leads to increased catalytic efficiency and the design of a potent inhibitor. The notion of a flexible linker between the sites, which must have more than a minimal number of residues, provides an explanation for recognition and discrimination against different substrates.

The transitions of the cell cycle are regulated by the cyclin dependent protein kinases (CDKs). The cyclins activate their respective CDKs and confer substrate recognition properties. We report the structure of phospho-CDK2/cyclin B and show that cyclin B confers M phase-like properties on CDK2, the kinase that is usually associated with S phase. Cyclin B produces an almost identical activated conformation of CDK2 as that produced by cyclin A. There are differences between cyclin A and cyclin B at the recruitment site, which in cyclin A is used to recruit substrates containing an RXL motif. Because of sequence differences this site in cyclin B binds RXL motifs more weakly than in cyclin A. Despite similarity in kinase structures, phospho-CDK2/cyclin B phosphorylates substrates, such as nuclear lamin and a model peptide derived from p107, at sequences SPXX that differ from the canonical CDK2/cyclin A substrate recognition motif, SPXK. CDK2/cyclin B phosphorylation at these non-canonical sites is not dependent on the presence of a RXL recruitment motif. The p107 peptide contains two SP motifs each followed by a non-canonical sequence of which only one site (Ser640) is phosphorylated by pCDK2/cyclin A while two sites are phosphorylated by pCDK2/cyclin B. The second site is too close to the RXL motif to allow the cyclin A recruitment site to be effective, as previous work has shown that there must be at least 16 residues between the catalytic site serine and the RXL motif. Thus the cyclins A and B in addition to their role in promoting the activatory conformational switch in CDK2, also provide differential substrate specificity.

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