DNA molecular wires have been studied extensively because of the ease with which molecules of controlled length and composition can be synthesized. The same has not been true for proteins. Here, we have synthesized and studied a series of consensus tetratricopeptide repeat (CTPR) proteins, spanning 4 to 20 nm in length, in increments of 4 nm. For lengths in excess of 6 nm, their conductance exceeds that of the canonical molecular wire, oligo­(phenylene-ethylenene), because of the more gradual decay of conductance with length in the protein. We show that, while the conductance decay fits an exponential (characteristic of quantum tunneling) and not a linear increase of resistance with length (characteristic of hopping transport), it is also accounted for by a square-law dependence on length (characteristic of weakly driven hopping). Measurements of the energy dependence of the decay length rule out the quantum tunneling case. A resonance in the carrier injection energy shows that allowed states in the protein align with the Fermi energy of the electrodes. Both the energy of these states and the long-range of hopping suggest that the reorganization induced by hole formation is greatly reduced inside the protein. We outline a model for calculating the molecular-electronic properties of proteins.

Summary Interactions of the chemokine CCL5 (RANTES) with glycosaminoglycans (GAGs) are crucial to the CCL5-mediated inflammation process. However, structural information on interactions between CCL5 and longer GAG fragments is lacking. In this study, the interactions between oligosaccharides derived from chondroitin sulfate and a dimeric variant of CCL5 were investigated using solution NMR. The data indicate that, in addition to the BBXB motif in the 40s loop, GAGs also contact residues in the N-loop in a manner similar to interactions between chemokine and the receptor N-terminus, and leading to possible stabilization of the dimer. Using TEMPO-tagged hexasaccharides, the binding orientation of the hexasaccharides was shown to be highly dependent on the sulfation pattern of the GalNAc groups. Finally, a model of the CCL5 dimer complexed to CS hexasaccharides was constructed using paramagnetic relaxation enhancement and intra- and inter-molecular NOEs constraints.

Pleiotrophin (PTN) is a potent glycosaminoglycan-binding cytokine important in neural development, angiogenesis and tissue regeneration. Much of its activity is attributed to its interactions with the chondroitin sulfate (CS) proteoglycan, receptor type protein tyrosine phosphatase ζ (PTPRZ). However, there is little high resolution structural information on interactions between PTN and CS, nor is it clear why the C-terminal tail of PTN is necessary for signaling through PTPRZ even though it does not contribute to binding heparin. We determined the first structure of PTN and analyzed its interactions with CS. Our structure shows PTN possesses large basic surfaces on both of its structured domains and residues in the hinge segment connecting the domains have significant contacts with the C-terminal domain. Our analysis of PTN-CS interactions showed the C-terminal tail of PTN is essential for maintaining stable interactions with CSA, the type of CS commonly found on PTPRZ. These results offer the first possible explanation of why truncated PTN missing the C-terminal tail is unable to signal through PTPRZ. NMR analysis of PTN’s interactions with CS revealed that the C-terminal domain and hinge of PTN make up the major CS binding site in PTN, and that removal of the C-terminal tail weakened the site’s affinity for CSA, but not for other high sulfation density CS.

Pleiotrophin (PTN) is a potent mitogenic cytokine whose activities are controlled by its interactions with glycosaminoglycan (GAG). We examined the specificity of PTN for several types of GAG oligosaccharides. Our data indicate that the interaction of PTN with GAGs is dependent on the sulfation density of GAGs. Surprisingly, an acidic peptide also had similar interactions with PTN as GAGs. This shows that the interaction of PTN with anionic polymers is flexible and adaptable and that the charge density is the main determinant of the interaction. In addition, we show that PTN can compensate for the loss of its termini in interactions with heparin oligosaccharides, allowing it to maintain its affinity for GAGs in the absence of the termini. Taken together, these data provide valuable insight into the interactions of PTN with its proteoglycan receptors.

As molecular electronic components, proteins are distinguished by a remarkably long electronic decay length (~10 nm) together with high contact resistance and extreme sensitivity to the chemical details of the contact. As a consequence, the conductance of even a large bioelectronic assembly is largely controlled by the conductance of the contacts. Streptavidin is a versatile linker-protein that can tether together biotinylated electrodes and biotinylated proteins, but with an ambiguity about the contact geometry that arises from its four possible binding sites for biotin. Here, we use engineered streptavidin tetramers, selected to contain a defined ratio of active monomers to 'dead' monomers so as to define the biotin binding sites. We find a strong dependence of conductance on the separation of the biotin molecules, consistent with a short-range tunneling interaction within the streptavidin, and in contrast to the long-range transport observed inside larger proteins. Hexaglutamate tails label the active monomers and the additional negative charge enhances conductance significantly. This effect is quantitatively accounted for by an electronic resonance in the protein conductance.