Chimeric contribution of human extended pluripotent stem cells to monkey embryos ex vivo

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Catch bonds: physical models, structural bases, biological function and rheological relevance

Force can shorten the lifetimes of macromolecular complexes (e.g., receptor-ligand bonds) by accelerating their dissociation. Perhaps paradoxical at first glance, bond lifetimes can also be prolonged by force. This counterintuitive behavior was named catch bonds, which is in contrast to the ordinary slip bonds that describe the intuitive behavior of lifetimes being shortened by force. Fifteen years after their theoretical proposal, catch bonds have finally been observed. In this article we review recently published data that have demonstrated catch bonds in the selectin system and suggested catch bonds in other systems, the theoretical models for their explanations, possible structural bases, their relation to flow-enhanced adhesion, and the potential biorheological relevance.     

The calmodulin binding region of the skeletal ryanodine receptor acts as a self-modulatory domain

A synthetic peptide (CaMBP) matching amino acids 3614-3643 of the skeletal ryanodine receptor (RyR1) binds to both Ca2+-free calmodulin (CaM) and Ca2+-bound CaM with nanomolar affinity [J. Biol. Chem. 276 (2001) 2069]. We report here that CaMBP increases [3H]ryanodine binding to RyR1 in a dose- and Ca2+-dependent manner; it also induces Ca2+ release from SR vesicles, and increases open probability (P(o)) of single RyR channels reconstituted in planar lipid bilayers. Further, CaMBP removes CaM associated with SR vesicles and increases [3H]ryanodine binding to purified RyR1, suggesting that its mechanism of action is two-fold: it removes endogenous inhibitors and also interacts directly with complementary regions in RyR1. Remarkably, the N-terminus of CaMBP activates RyRs while the C-terminus of CaMBP inhibits RyR activity, suggesting the presence of two discrete functional subdomains within this region. A ryr1 mutant lacking this region, RyR1-Delta3614-3643, was constructed and expressed in dyspedic myoblasts (RyR1-knockout). The depolarization-, caffeine- and 4-chloro-m-cresol (4-CmC)-induced Ca2+ transients in these cells were dramatically reduced compared with cells expressing wild type RyR1. Deletion of the 3614-3643 region also resulted in profound changes in unitary conductance and channel gating. We thus propose that the RyR1 3614-3643 region acts not only as the CaM binding site, but also as an important modulatory domain for RyR1 function.     

不同光强下高锰对黄瓜光合作用特性的影响

采用营养液培养的方法,研究了不同光强下高锰对黄瓜植株生长、叶绿素含量、叶绿素荧光参数和光合作用的影响.结果表明,高锰处理抑制了黄瓜植株的生长,与弱光处理相比强光下抑制幅度更加显著.强光下,高锰处理显著降低叶绿素含量,但降低光强却增加其含量.强光下,高锰处理显著降低原初光能转换效率(F_v/F_m)、光合电子传递量子效率(ΦPSII)和光化学猝灭系数(qP);弱光下,高锰处理对F_v/F_m和qP无显著影响.高锰处理使净光合速率(P_n)和气孔导度(G_s)下降.尤其是在强光下下降幅度更大.高锰处理使细胞间CO₂浓度(C_i)在强光下升高,而在弱光下则下降.与C_i相反,高锰处理使气孔限制值(L_s)在强光下下降,而在弱光下上升.因此,强光下高锰胁迫使净光合速率下降可能是由非气孔限制引起的,而弱光下高锰处理使净光合速率下降可能是由气孔因子限制引起的.     

Threonine-insensitive Homoserine Dehydrogenase from Soybean: GENOMIC ORGANIZATION, KINETIC MECHANISM, AND IN VIVO ACTIVITY*

Aspartate kinase (AK) and homoserine dehydrogenase (HSD) function as key regulatory enzymes at branch points in the aspartate amino acid pathway and are feedback-inhibited by threonine. In plants the biochemical features of AK and bifunctional AK-HSD enzymes have been characterized, but the molecular properties of the monofunctional HSD remain unexamined. To investigate the role of HSD, we have cloned the cDNA and gene encoding the monofunctional HSD (GmHSD) from soybean. Using heterologously expressed and purified GmHSD, initial velocity and product inhibition studies support an ordered bi bi kinetic mechanism in which nicotinamide cofactor binds first and leaves last in the reaction sequence. Threonine inhibition of GmHSD occurs at concentrations (K_i = 160–240 mm) more than 1000-fold above physiological levels. This is in contrast to the two AK-HSD isoforms in soybean that are sensitive to threonine inhibition (K_i∼150 μm). In addition, GmHSD is not inhibited by other aspartate-derived amino acids. The ratio of threonine-resistant to threonine-sensitive HSD activity in soybean tissues varies and likely reflects different demands for amino acid biosynthesis. This is the first cloning and detailed biochemical characterization of a monofunctional feedback-insensitive HSD from any plant. Threonine-resistant HSD offers a useful biotechnology tool for manipulating the aspartate amino acid pathway to increase threonine and methionine production in plants for improved nutritional content.