In vitro synthesized bacterial outer membrane protein is integrated into bacterial inner membranes but translocated across microsomal membranes

The LamB protein is an integral membrane protein of the outer membrane of Escherichia coli. We have now found that, when synthesized in an E. coli cell-free translation system supplemented with inverted vesicles derived from the E. coli inner membrane, LamB protein is integrated into the vesicle membrane as assayed by its resistance to extraction at alkaline pH. These data suggest that the inner membrane is the primary site for integration of LamB protein prior to subsequent sorting to the outer membrane. When synthesized in a wheat germ cell-free translation system supplemented with canine microsomal membranes, LamB protein is glycosylated at one or two cryptic sites, and surprisingly, it is translocated across instead of being integrated into the vesicle membrane. We suggest that the translocation machinery of the microsomal membrane, although able to recognize the signal sequence(s) of LamB, is unable to recognize its stop-transfer sequence(s), thereby yielding translocation instead of integration.     

Protein translocation across the eukaryotic endoplasmic reticulum and bacterial plasma membranes

A decisive step in the biosynthesis of many proteins is their partial or complete translocation across the eukaryotic endoplasmic reticulum membrane or the prokaryotic plasma membrane. Most of these proteins are translocated through a protein-conducting channel that is formed by a conserved, heterotrimeric membrane-protein complex, the Sec61 or SecY complex. Depending on channel binding partners, polypeptides are moved by different mechanisms: the polypeptide chain is transferred directly into the channel by the translating ribosome, a ratcheting mechanism is used by the endoplasmic reticulum chaperone BiP, and a pushing mechanism is used by the bacterial ATPase SecA. Structural, genetic and biochemical data show how the channel opens across the membrane, releases hydrophobic segments of membrane proteins laterally into lipid, and maintains the membrane barrier for small molecules.     

Analysis of the effects of mass transfer on heat transfer in the process of moisture exchange across a membrane

A theoretical study was conducted to investigate the effects of mass transfer on heat transfer in the process of moisture exchange across a membrane and a mathematical model describing the heat transfer with consideration of the effect of mass transfer was developed.A dimensionless variable,ψi,which presents the degree to which the mass transfer affects the heat transfer,was proposed through theoretical analysis.With calculating of this dimensionless variable,the heat transfer coupled with mass transfer can...     

Translocation of lipid-linked oligosaccharides across the ER membrane requires Rft1 protein.

N-linked glycosylation of proteins in eukaryotic cells follows a highly conserved pathway. The tetradecasaccharide substrate (Glc3Man9GlcNAc2) is first assembled at the membrane of the endoplasmic reticulum (ER) as a dolichylpyrophosphate (Dol-PP)-linked intermediate, and then transferred to nascent polypeptide chains in the lumen of the ER. The assembly of the oligosaccharide starts on the cytoplasmic side of the ER membrane with the synthesis of a Man5GlcNAc2-PP-Dol intermediate. This lipid-linked intermediate is then translocated across the membrane so that the oligosaccharides face the lumen of the ER, where the biosynthesis of Glc3Man9GlcNAc2-PP-Dol continues to completion. The fully assembled oligosaccharide is transferred to selected asparagine residues of target proteins. The transmembrane movement of lipid-linked Man5GlcNAc2 oligosaccharide is of fundamental importance in this biosynthetic pathway, and similar processes involving phospholipids and glycolipids are essential in all types of cells. The process is predicted to be catalysed by proteins, termed flippases, which to date have remained elusive. Here we provide evidence that yeast RFT1 encodes an evolutionarily conserved protein required for the translocation of Man5GlcNAc2-PP-Dol from the cytoplasmic to the lumenal leaflet of the ER membrane.     

The membrane attachment protein for spectrin is associated with band 3 in human erythrocyte membranes.

Ankyrin, the membrane attachment protein for human erythrocyte spectrin, is tightly linked in a 1:1 molar ratio with band 3 in detergent extracts of spectrin-depleted membranes. Ankyrin-linked band 3, which represents 10--15% of the total band 3, spans the membrane, and is nearly identical to the major band 3 by peptide analysis. Spectrin binds to solubilised ankyrin-linked band 3, but not to free band 3. A portion of band 3 remains firmly associated with detergent-extracted cytoskeletal proteins. It is concluded that a fraction of band 3 is attached to the erythrocyte cytoskeleton through association with ankyrin, which in turn is bound to spectrin.     

液膜法处理工业废水传质机理的探讨

本文通过液膜法处理工业废水的传质机理研究,揭示了液膜分离具有高效,快速等特点,并通过实例分析,为液膜分离实现工业化提供了理论依据。     

西北产贯叶连翘挥发性化学成分研究

目的为了研究西北产贯叶连翘挥发性化学成分。方法采用传统水蒸汽蒸馏法提取贯叶连翘全草挥发油,并通过气相色谱/质谱/计算机联用技术测定了提取物的化学成分。结果从中鉴定出63种化合物,结果显示氧化石竹烯(6.94%)、环十二烷(3.67%)、苄醇(2.14%)、斯巴醇(3.36%)、环十四烷(1.96%)、苯并呋喃酮(1.30%)、喇叭茶醇(1.32%)等化合物为贯叶连翘挥发油的主要成分。结论西北地区的贯叶连翘挥发油成分与文献报道的其他地区产贯叶连翘挥发油成分有很大不同,说明地域和气候环境对植物的成分组成有很大影响。     

叶面施硼对蚕豆膜脂过氧化作用及膜保护系统的影响

蚕豆不同生长期用不同浓度的硼酸溶液进行叶面喷施．结果表明，在幼苗期、现蕾期和开花期喷施不同浓度硼酸溶液，能增强蚕豆叶片的SOD活性和CAT活性，降低O2^-生成速率和MDA含量．于开花期前喷施0．1％-0.2％的硼酸溶液最为有效．     

迷迭香挥发性成分的质谱分析

由黔产迷失香（Rosmarinus officinalisL.）的全草提取挥发油，用气相色谱－质谱进行分离测定，结合计算机检索技术对分离化合物进行结构鉴定，应用色谱峰面积归一化测定各成分的相对百分含量。鉴定出29个化学成分。主要成分为α－蒎烯、莰烯、α－水芹烯、β－水芹烯、1，8－桉叶素、樟脑、龙脑、乙酸龙脑酯、α－石竹烯、和β－石竹烯等。     

南海海洋真菌2492号中的甾体成分

对南海海洋真菌2492分离自香港红树林植物Phramites australi，从其菌丝体的乙醇粗提物分离出3个甾体化合物：(22E，24R)—麦角甾—5，7，22—三烯—3β醇；5a，8a—表二氧—(22E，24R)—麦角甾—6，22—二烯—3β醇和(22E，24R)—24—甲基麦角甾—7，22—三烯—3β，5a，6a—三醇。它们的结构通过IR，FABMS和NMR谱图得到确定。