Theoretical Studies on the Mechanism of the Proton-transfer-coupled Electron Transfer Reactions Between Menaquinone Q_A and Ubiquinone Q_B in the Bacterial Photosynthetic Reaction Center of Rhodopseudomona viridis

The mechanism of the proton_transfer_coupled electron transfer (PT_ET) reactions between the menaquinone Q A (MQ 1) and ubiquinone Q B (UQ 1) in the bacterial photosynthetic reaction center of Rhodopseudomona viridis was studied by using the B3LYP/6_31G(d) method. The changes of standard Gibbs free energy ΔG 0 of all possible reactions followed the ET reaction (1) were calculated. The results indicated that: (1) according to the ΔG 0 values of corresponding reactions, UQ 1 could not accept two electrons from MQ - 1 continually without the coupled proton transfer reactions. Because of ΔG 0 2b 0, ΔG 0 3b 0 and ΔG 0 4b 0, the corresponding PT_ET reactions could take place along with reactions (2b), (3b) and (4b) sequentially; (2) on the gaseous condition, the first and second transferred protons (H +(1) and H +(2)) from the surrounding amino acid residues or water molecules will combine with the oxygen No.7 and oxygen No.8 of UQ 1, respectively. On the condition of protein surroundings (by SCRF model, ε =4.0), the results are converse but the energy difference between the combination of H +(1) and H +(2) with UQ - 1 is quite small. The difference of ΔG 0 values between the corresponding reactions in gaseous surroundings and the SCRF model is not significant; (3) the PT_ET reactions between MQ 1 - and UQ 1 - should be as follows: MQ 1 -+UQ 1→MQ 1+UQ 1 - (1) UQ 1 - ( O (7) )+H +( HisL 190)→UQ 1H(2b) ( Gas ) or UQ 1 - ( O (8) )+H +(H 2O)→UQ 1H (2b') ( SCRF ) or UQ 1 - ( O (8) )+H + ( ArgL 217)→UQ 1H(2b') ( SCRF ) MQ 1 -+UQ 1H→MQ 1+UQ 1H - (3b) ( Gas ) MQ 1 -+UQ 1H→MQ 1+UQ 1H -(3b') ( SCR F) UQ 1H -+H +(H 2O)→UQ 1H 2(4b) ( Gas ) or UQ 1H -+H + ( ArgL 217)→UQ 1H 2 (4b) ( Gas ) or UQ 1H -+H + ( HisL 190)→UQ 1H 2 (4b') ( SCRF )

Calmodulin regulates the post-anaphase reposition of centrioles during cytokinesis

A transient postanaphase repositioning of the centriole is found to control the completion of cytokinesis. Using agreen fluorescent protein-calmodulin fusion protein as a living cell probe, we have previously found that calmodulin isassociated with the initiation and progression of cytokinesis. In this study, we further studied the effect of calmodulinon the repositioning of the centriole and subsequent cell cycle progression. When activity of calmodulin is inhibited, theregression of the centriole from the intercellular bridge to the cell center is blocked, and thus the completion of celldivision is repressed and two daughter cells are linked by longer cell bridge in perturbed cells. W7 treatment duringcytokinesis also results in unfinished cytokinesis and stopped G1 phase. These results suggest that calmodulin activity isrequired for centriole repositioning and can affect the completion of cytokinesis and cell cycle progression.

Pericentrin contains five NESs and an NLS essential for its nucleocytoplasmic trafficking during the cell cycle

Pericentrin, a conserved centrosomal component, provides the structural scaffold to anchor numerous centrosomal proteins, and thus plays an essential role in the organization and function of the centrosome and the mitotic spindle. Although pericentrin was shown to localize in the cytoplasm and reported to be sensitive to leptomycin B （LMB）, a specific inhibitor of Crml, the regions within pericentrin that serve as signals for transporting in and out of the nucleus have not yet been identified. In this study, we identified five novel nuclear export signals （NESs） in pericentrin with diverse export activities. All of the five NESs could bind to Crml in a LMB-sensitive way when mediating the nuclear export of pericentrin. We also demonstrated that the region of amino acids 8-42 in pericentrin contains a tripartite nuclear localization signal （NLS） consisting of three clusters of basic amino acids. The NLS of pericentrin binds to importin β directly or via the adaptor importin α to form the import complex, which could be disrupted by RanQ69L, a dominant-negative Ran GTPase possessing high affinity for importin β. Furthermore, we found that mutation of the NESs in full-length pericentrin results in both nuclear and cytoplasmic localization, and mutation of the NLS abolishes the nuclear import of pericentrin. On the basis of these results, we suggest that the NESs and NLS of pericentrin are essential for its subcellular localization and nucleocytoplasmic trafficking during the cell cycle.

Potential existence of two independent centrosome-targeting domains in PP4

Protein phosphatase 4 (PP4) is an important member in the PPP family of protein Ser/Thr phosphatases. It has been proven to regulate a variety of cellular processes such as centrosome maturation, micro- tubule nucleation, splicesome assembly, and JNK pathway activation. Compared to the crystallized and structurally well defined phosphatase PP1 and PP2B, little is known about the structure of PP4. Besides the conserved motifs characteristic of the PPP family, no information is available on the other domains of PP4. PP4 is reported to localize to the centrosome in many species such as Drosophila, Caenor- habditis elegans and mammalian cells, which suggests a conserved role of PP4 in the regulation of centrosome function. Unlike several other centrosomal proteins, no sequence has been identified for PP4 that can target it to specific centrosomal localization. In this study, we used a combination of PCR mutagenesis and transient expression of GFP-tagged proteins in mammalian cells, and identified two PP4 centrosome-targeting domains of 68―136 and 134―220 aa. These two domains may be associated for appropriate localization to the centrosome. The findings are useful for further elucidating the func- tion of the domains and other structural characteristics of PP4.