Here, we present a structural and dynamic description of CBP-ID4 at atomic resolution. ID4 is the fourth intrinsically disordered linker of CREB-binding protein (CBP). In spite of the largely disordered nature of CBP-ID4, NMR chemical shifts and relaxation measurements show a significant degree of α-helix sampling in the protein regions encompassing residues 2-25 and 101-128 (1852-1875 and 1951-1978 in full-length CBP). Proline residues are uniformly distributed along the polypeptide, except for the two α-helical regions, indicating that they play an active role in modulating the structural features of this CBP fragment. The two helical regions are lacking known functional motifs, suggesting that they represent thus-far uncharacterized functional modules of CBP. This work provides insights into the functions of this protein linker that may exploit its plasticity to modulate the relative orientations of neighboring folded domains of CBP and fine-tune its interactions with a multitude of partners.

eThe purple bacterium Rhodopseudomonas palustris TIE-1 expresses multiple small high-potential redox proteins during photoautotrophic growth, including two high-potential iron-sulfur proteins (HiPIPs) (PioC and Rpal_4085) and a cytochrome c 2 . We evaluated the role of these proteins in TIE-1 through genetic, physiological, and biochemical analyses. Deleting the gene encoding cytochrome c 2 resulted in a loss of photosynthetic ability by TIE-1, indicating that this protein cannot be replaced by either HiPIP in cyclic electron flow. PioC was previously implicated in photoferrotrophy, an unusual form of photosynthesis in which reducing power is provided through ferrous iron oxidation. Using cyclic voltammetry (CV), electron paramagnetic resonance (EPR) spectroscopy, and flash-induced spectrometry, we show that PioC has a midpoint potential of 450 mV, contains all the typical features of a HiPIP, and can reduce the reaction centers of membrane suspensions in a light-dependent manner at a much lower rate than cytochrome c 2 . These data support the hypothesis that PioC linearly transfers electrons from iron, while cytochrome c 2 is required for cyclic electron flow. Rpal_4085, despite having spectroscopic characteristics and a reduction potential similar to those of PioC, is unable to reduce the reaction center. Rpal_4085 is upregulated by the divalent metals Fe(II), Ni(II), and Co(II), suggesting that it might play a role in sensing or oxidizing metals in the periplasm. Taken together, our results suggest that these three small electron transfer proteins perform different functions in the cell.

Biological macromolecules involved in electron transfer reactions display chains of closely packed redox cofactors when long distances must be bridged. This is a consequence of the need to maintain a rate of transfer compatible with metabolic activity in the framework of the exponential decay of electron tunneling with distance. In this work intermolecular electron transfer was studied in kinetic experiments performed with the small tetraheme cytochrome from Shewanella oneidensis MR-1 and from Shewanella frigidimarina NCIMB400 using non-physiological redox partners. This choice allowed the effect of specific recognition and docking to be eliminated from the measured rates. The results were analyzed with a kinetic model that uses the extensive thermodynamic characterization of these proteins reported in the literature to discriminate the kinetic contribution of each heme to the overall rate of electron transfer. This analysis shows that, in this redox chain that spans 23 Å , the kinetic properties of the individual hemes establish a functional specificity for each redox center. This functional specificity combined with the thermodynamic properties of these soluble proteins ensures directional electron flow within the cytochrome even outside of the context of a functioning respiratory chain.Shewanella are facultative anaerobic ␥-proteobacteria capable of reducing a multitude of organic and inorganic substrates, including soluble and insoluble metallic compounds containing iron, manganese, uranium, or chromium (1). Shewanella arouse widespread interest in the science and in the engineering communities due to their role in geological phenomena such as global weathering and formation of minerals, their possible application in bioremediation of contaminated environments polluted with heavy metals, and their biotechnological applications for energy production in microbial fuel cells (1-3).The anaerobic respiratory flexibility found in these bacteria is associated with the presence of numerous multiheme cytochromes (4). One of the most abundant is the small tetraheme cytochrome (STC) 2 that has a 12-kDa molecular mass and contains four c-type hemes (5). Although the physiological role of STC in Shewanella oneidensis MR-1 (SoSTC) is still unknown, in S. frigidimarina NCIMB400 (SfSTC) it was shown to participate in iron respiration (6). The three-dimensional structure of SoSTC was determined for the reduced and oxidized states by x-ray crystallography (7), and recently the solution structure was solved for the reduced state of SfSTC (8). These studies showed that, despite the diversity found in the amino acid sequence of these cytochromes (64% identical), the architecture of their heme core is highly conserved with the hemes organized in a chain spanning 23 Å for the most distant heme irons (7,8). The determination of the structure of SoSTC led to the proposal that this protein may work as a nonspecific electron harvester (7). This proposal was refined by considering that hemes I-III would feed electrons to heme IV on t...

The E7 protein from human papillomavirus (HPV) plays a key role in oncogenesis; for this reason, it is a target of great biomedical interest. To date, no high resolution information is available for the full protein. We present here the NMR characterization of the entire E7 from HPV16, one of the most oncogenic variants of the virus. The protein is very heterogeneous in terms of structural and dynamic properties with a highly flexible N-terminal module and a more structured C terminus. This opens possibilities for studies of molecular-level interactions and post-translational modifications of the protein to unravel functional details that might be linked to its highly oncogenic potential.