Crystal structure of CyanoQ from the thermophilic cyanobacterium Thermosynechococcus elongatus and detection in isolated photosystem II complexes.

The PsbQ-like protein, termed CyanoQ, found in the cyanobacterium Synechocystis sp. PCC 6803 is thought to bind to the lumenal surface of photosystem II (PSII), helping to shield the Mn(4)CaO(5) oxygen-evolving cluster. CyanoQ is, however, absent from the crystal structures of PSII isolated from thermophilic cyanobacteria raising the possibility that the association of CyanoQ with PSII might not be a conserved feature. Here, we show that CyanoQ (encoded by tll2057) is indeed expressed in the thermophilic cyanobacterium Thermosynechococcus elongatus and provide evidence in support of its assignment as a lipoprotein. Using an immunochemical approach, we show that CyanoQ co-purifies with PSII and is actually present in highly pure PSII samples used to generate PSII crystals. The absence of CyanoQ in the final crystal structure is possibly due to detachment of CyanoQ during crystallisation or its presence in sub-stoichiometric amounts. In contrast, the PsbP homologue, CyanoP, is severely depleted in isolated PSII complexes. We have also determined the crystal structure of CyanoQ from T. elongatus to a resolution of 1.6 Å. It lacks bound metal ions and contains a four-helix up-down bundle similar to the ones found in Synechocystis CyanoQ and spinach PsbQ. However, the N-terminal region and extensive lysine patch that are thought to be important for binding of PsbQ to PSII are not conserved in T. elongatus CyanoQ. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s11120-014-0010-z) contains supplementary material, which is available to authorized users

The generation of distortion products in a nonlinear transmission line model of the cochlea

A method is presented to study the generation of intermodulation distortion products (IDPs) in a nonlinear model of the human cochlea in detail. The model used is a simple one-dimensional (long wave) transmission line model, using a simple mass-stiffness-damping combination for the local cochlear partition mechanics. Nonlinearity is introduced in the damping term, causing the generation of IDPs. First it is shown that these IDPs have properties in common with experimental data, measured both psychophysically and in otoacoustic emissions. Although there are clear (quantitative) differences with the experimental data, some of the (qualitative) behavior is similar. The method presented here for studying the generation process in more detail assumes that the total excitation at any section of the model and at a certain (IDP) frequency can be regarded as the sum of contributions from all model sections where distortion generation takes place. The resulting patterns of generated distortion and of contributions to emission and excitation at the characteristic position (DP-place) of the 2f(1)-f(2) component explain some of the behavior of this IDP component. It is shown that the generation at the DP-place is influenced by reflection of energy at the stapes boundary, whereas in the case of the emission the phase differences of contributions from the main generation region play a crucial role