Igor Kaltashov and Group Designs a Bio-Trap

The emergence of nanomedicine and molecular therapeutics in recent years has resulted in a surge of interest in designing macromolecules with precisely controlled properties. Although the majority of work in this field is now focused on nanocarriers as drug delivery vehicles, significant efforts are also invested toward designing constructs inspired by protein cage architectures, such as viral capsids. The approach presented in a recent publication from the Kaltashov laboratory (Sjoelund and Kaltashov, Biochemistry, 46 (46), 13382 -13390, 2007) is likely to catalyze further developments in this field by inspiring and guiding design of nano-devices for multiple tasks ranging from sequestering small-molecule toxins in both tissue and circulation to nutrient deprivation of pathogens.

Figure 1 Hypothetical structures of CRABP I A35C/T57C built on the CRABP I backbone (1CBI for the apo form and 1CBR for the holo form). (Left) Superimposed backbone traces of the apo (cyan) and holo (gray) conformations of the protein. (Center) Surface model of apo-CRABP I A35C/T57C with the introduced cysteine residues shown in orange. (Right) Model of holo-CRABP I A35C/T57C with the cysteine residues shown in orange and RA shown in green.

The researchers designed a bio-trap using cellular retinoic acid binding protein I (CRABP I), where ligand entry to, and exit from, the internal binding site occurs via a flexible portal region, functioning as a dynamic aperture. Introduction of two cysteine amino acids in strategic locations results in formation of a disulfide bond following ligand entry to the internal binding site. This covalent bond irreversibly seals the dynamic aperture and effectively locks the ligand inside the binding cavity. The disulfide bond formation can only be triggered by the ligand binding event, and the double mutant behaves just as a wild type protein in the absence of retinoic acid. Since the release of the ligand and recycling of the "transporter-turned-trap" protein is only possible under reducing conditions, systems similar to the one presented in this work may also be used in biotechnological applications that require highly efficient mono-directional transport of small molecules (e.g., bioreactor-based manufacturing).

November 2007