Introduction

The Editors wish to thank Dr. G. Daum for coordinating this multi-author review.     

The reversibility of mitotic exit in vertebrate cells

A guiding hypothesis for cell-cycle regulation asserts that regulated proteolysis constrains the directionality of certain cell-cycle transitions. Here we test this hypothesis for mitotic exit, which is regulated by degradation of the cyclin-dependent kinase 1 (Cdk1) activator, cyclin B. Application of chemical Cdk1 inhibitors to cells in mitosis induces cytokinesis and other normal aspects of mitotic exit, including cyclin B degradation. However, chromatid segregation fails, resulting in entrapment of chromatin in the midbody. If cyclin B degradation is blocked with a proteasome inhibitor or by expression of non-degradable cyclin B, Cdk inhibitors will nonetheless induce mitotic exit and cytokinesis. However, if after mitotic exit, the Cdk1 inhibitor is washed free from cells in which cyclin B degradation is blocked, the cells can revert back to M phase. This reversal is characterized by chromosome recondensation, nuclear envelope breakdown, assembly of microtubules into a mitotic spindle, and in most cases, dissolution of the midbody, reopening of the cleavage furrow, and realignment of chromosomes at the metaphase plate. These findings demonstrate that proteasome-dependent degradation of cyclin B provides directionality for the M phase to G1 transition.     

Lipid transport in microorganisms

Summary Microorganisms are useful model systems for the study of intracellular transport of lipids. Eukaryotic microorganisms, such as the yeastSaccharomyces cerevisiae, are similar to higher eukaryotes with respect to organelle structure and membrane assembly. Experiments in vivo showed that transport of phosphatidylcholine between yeast microsomes and mitochondria is energy independent; transfer of phosphatidylinositol to the plasma membrane and the flux of secretory vesicles take place by different mechanisms. Linkage of transfer and biosynthesis of phospholipids was demonstrated in the case of intramitochondrial phospholipid transfer. A yeast phosphatidylinositol/phosphatidylcholine transfer protein, which is essential for cell viability, was isolated and characterized. Another phospholipid transfer protein present in yeast cytosol, which has a different specificity, is currently under investigation. Transfer of phospholipids between cellular membranes was also demonstrated with prokaryotes. The cytoplasm and the periplasma of the gram-negative facultative photosynthetic bacteriumRhodopseudomonas sphaeroides contain phospholipid transfer proteins; these seem to be involved in the biosynthesis of prokaryotic membranes.     

爱因斯坦A-Z

本书是一本传记性著作，记述了20世纪最伟大的科学家之一爱因斯坦的生平。与其他传记体文学不同的是，本书别出心裁的以字母表的顺序，以关键词的方式记录了爱因斯坦的生活、科研、理想等等。从第一个单词大大咧咧的（Absentmindeness），一直到最后一个单词犹太复国主义（Zionism）。每个单词作为一章的标题引出了一段或惊险、或平淡的故事。从本书中读者可以了解爱因斯坦在科学方面的天才想法，可以读到他作为普通人生活的一面——一个可怜的丈夫和父亲，还有他在核武器方面的影响，他创造了相对论却拒绝接受量子力学……