Biomolecular motors have inspired the design and construction of artificial nanoscale motors and machines based on nucleic acids, small molecules, and inorganic nanostructures. However, the high degree of sophistication and efficiency of biomolecular motors, as well as their specific biological function, derives from the complexity afforded by protein building blocks. Here, we discuss a novel bottom-up approach to understanding biological motors by considering the construction of synthetic protein motors. Specifically, we present a design for a synthetic protein motor that moves along a linear track, dubbed the "Tumbleweed." This concept uses three discrete ligand-dependent DNA-binding domains to perform cyclically ligand-gated, rectified diffusion along a synthesized DNA molecule. Here we describe how de novo peptide design and molecular biology could be used to produce the Tumbleweed, and we explore the fundamental motor operation of such a design using numerical simulations. The construction of this and more sophisticated protein motors is an exciting challenge that is likely to enhance our understanding of the structure-function relationship in biological motors.

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Superantigens are a class of microbial proteins with the ability to excessively activate T cells by binding to the T cell receptor. The staphylococcal and streptococcal superantigens are closely related in structure and possess an N-terminal domain that resembles an OB fold and a C-terminal domain similar to a beta-grasp fold. Yersinia pseudotuberculosis produces superantigens, YPMa, YPMb, and YPMc, which have no significant amino acid similarity to other proteins. We have determined the crystal and solution structures of YPMa, which show that the protein has a jelly-roll fold. The closest structural neighbors to YPMa are viral capsid proteins and members of the tumor necrosis factor superfamily. In the crystal structure, YPMa packs as a trimer, another feature shared with viral capsid proteins and TNF superfamily proteins. However, in solution YPMa behaves as a monomer, and any functional relevance of the trimer observed in the crystals is yet to be established.

Acetohydroxy acid reductoisomerase (EC 1.1.1.86, AHAR) was purified to a high degree from green shoots of wheat (Triticum aestivum L. cv. Vulcan). The enzyme was localised in the chloroplasts, and activity was at a maximum approximately 4 d after germination. The subunit molecular mass of wheat AHAR was 57 kD and activity of the native enzyme had an elution volume from size exclusion columns that corresponded to a molecular mass of 47 kD. The enzyme did not require the addition of Mg 2+ ions to reaction mixtures for activity. The Km values for (R,S)-2-acetolactate and (R,S)-2-aceto-2-hydroxybutyrate were 91 and 9 mM, respectively, and the corresponding maximum velocities were 430 and 451 mU mg –1 protein. The Km for NADPH was approximately 10 mM when either of the acetohydroxy acids was the other substrate. Preparation of the acetohydroxy acid substrates by hydrolysis of the parent esters in strong base led to the formation of inhibitory by-products. Racemisation of the acetohydroxy acids was detected in assay mixtures.

Yersinia pseudotuberculosis-derived mitogen (YPM), a superantigen with no amino-acid sequence similarity to other known superantigens, has been crystallized by the sitting-drop vapour-diffusion method. The crystals belong to space group C2, with unit-cell parameters a = 138.67, b = 78.66, c = 32.91 A, beta = 91.97 degrees. A native data set has been collected to a resolution of 1.8 A using synchrotron radiation. Self-rotation function calculations suggest the presence of three molecules in the asymmetric unit, corresponding to a solvent content of 45%.