Turgor-regulated translocation

Abstract. The role played by potassium in plants is examined, in particular its connection with phloem translocation. The possibility emerges that trans-location may be a turgor-regulated process in which potassium plays a central role.
A simple mechanistic hypothesis is proposed and the evidence for it is discussed. The hypothesis is strongly supported by both the direct and the circumstantial evidence.
An important conclusion is the need to reassess the importance of the translocation process in the control of assimilate partitioning. Hitherto, as implied by Münch's hypothesis, control has been assumed to be exerted by source and/or sink activity with the translocation pathway playing a more passive secondary role. If the present proposal is correct the translocation process emerges instead as a dominant factor in the control of assimilate partitioning.     

Bacteriophage translocation

The occurrence of phages in the human body, especially in the gastrointestinal tract, raises the question of their potential role in the physiology and pathology of this system. Especially important is the issue of whether phages can pass the intestinal wall and migrate to lymph, peripheral blood, and internal organs and, if so, the effects such a phenomenon could have (such passage by bacteria, known as bacterial translocation, has been shown to cause various disturbances in humans, from immune defects to sepsis). Available data from the literature support the assumption that phage translocation can take place and may have some immunomodulatory effects. In addition, phages of the gut may play a protective role by inhibiting local immune reactions to antigens derived from gut flora.     

Chaperone-assisted translocation

We investigate the translocation of a stiff polymer through a nanopore in a membrane, in the presence of binding particles (chaperones) that bind reversibly to the polymer on both sides of the membrane. A bound chaperone covers one (univalent binding) or many (multivalent binding) binding sites. Assuming that the diffusion of the chaperones is fast compared to the rate of translocation we describe the process by a one-dimensional master equation. We expand previous models by a detailed study of the effective force in the master equation, which is obtained by the appropriate statistical mechanical average over the chaperone states. The dependence of the force on the degree of valency (the number of binding sites occupied by a chaperone) is studied in detail. We obtain finite size corrections (to the thermodynamical expression for the force), which, for univalent binding, can be expressed analytically. We finally investigate the mean velocity for translocation as a function of chaperone binding strength and size. For both univalent and multivalent binding simple results are obtained for the case of a sufficiently long translocating polymer.     

Nanoelectropulse-induced phosphatidylserine translocation

Nanosecond, megavolt-per-meter, pulsed electric fields induce phosphatidylserine (PS) externalization, intracellular calcium redistribution, and apoptosis in Jurkat T-lymphoblasts, without causing immediately apparent physical damage to the cells. Intracellular calcium mobilization occurs within milliseconds of pulse exposure, and membrane phospholipid translocation is observed within minutes. Pulsed cells maintain cytoplasmic membrane integrity, blocking propidium iodide and Trypan blue. Indicators of apoptosis-caspase activation and loss of mitochondrial membrane potential-appear in nanoelectropulsed cells at later times. Although a theoretical framework has been established, specific mechanisms through which external nanosecond pulsed electric fields trigger intracellular responses in actively growing cells have not yet been experimentally characterized. This report focuses on the membrane phospholipid rearrangement that appears after ultrashort pulse exposure. We present evidence that the minimum field strength required for PS externalization in actively metabolizing Jurkat cells with 7-ns pulses produces transmembrane potentials associated with increased membrane conductance when pulse widths are microseconds rather than nanoseconds. We also show that nanoelectropulse trains delivered at repetition rates from 2 to 2000 Hz have similar effects, that nanoelectropulse-induced PS externalization does not require calcium in the external medium, and that the pulse regimens used in these experiments do not cause significant intra- or extracellular Joule heating.     

204 Polymer translocation

The translocation of a single macromolecule through a protein pore or a solid-state nanopore involves three major stages: (1) approach of the macromolecule towards the pore, (2) capture/recognition of the macromolecule at the pore entrance, and (3) threading through the pore (see the Figure) (Muthukumar, 2011). All of these stages are controlled by conformational entropy of the macromolecule, charge decoration, and the geometry of the pore, hydrodynamics, and electrostatic interactions. Chief among the contributing factors are the entropic barrier presented by the pore to the penetration of the macromolecule, pore–polymer interactions, electro-osmotic flow, and the drift-diffusion of the macromolecule in electrolyte solutions. A unifying theory of these contributing factors will be described in the context of a few illustrative experimental data on DNA translocation and protein translocation through protein pores and solid-state nanopores. Future challenges to specific biological systems will be briefly discussed.     

Apetiolar photosynthate translocation

Apetiolar transport of photosynthate ⁻¹⁴C has been studied by feeding of ¹⁴CO₂ to soybean petioles. Translocation occurs in the absence of leaves, but both the rate and velocity are diminished. The effect of root excision is not as profound as that of leaves. It appears, in some instances, to inhibit transport partially, so that accumulation of photosynthate develops, giving a steeper isotopic gradient. The effect of leaf darkening is to diminish its uptake of photosynthate from the petiole, possibly as a result of decreased transpiration in the lowered temperature of the darkened leaf. The data suggest that neither mass flow nor active transport provide an adequate basis for normal photosynthate transport but that the leaves provide a direct force requiring structural continuity, or a translocation carrier.     

生物大分子的细胞核质转运

生物大分子通过细胞核孔复合体的转运是真核细胞基因复制、转录和翻译的必要环节 ,也是联系细胞核内外信号转递与参与细胞内核反应 (即细胞增殖、分化、凋亡等核反应 )调控的重要环节。本文主要介绍细胞核孔复合体结构、出入细胞核的转运过程及核转运蛋白与亲核素方面的研究进展 ,细胞核转运过程的深入研究在医药学基础和临床实践都有十分重要的意义。     

Nucleolus organizer region--centromere translocation

A karyotype 45,XX,-13,-15,+psudic (13;15)(p12.9;11.200) was observed in a young woman after two spontaneous miscarriages. After R-, C-, and NOR - banding - the rearranged element was shown to include: the long arm, the active centromere, and the NOR of chromosome 13, followed by the inactivated centromere, and the long arm of chromosome 15.     

Proton translocation by transhydrogenase

Transhydrogenase, in animal mitochondria and bacteria, couples hydride transfer between NADH and NADP(+) to proton translocation across a membrane. Within the protein, the redox reaction occurs at some distance from the proton translocation pathway and coupling is achieved through conformational changes. In an 'open' conformation of transhydrogenase, in which substrate nucleotides bind and product nucleotides dissociate, the dihydronicotinamide and nicotinamide rings are held apart to block hydride transfer; in an 'occluded' conformation, they are moved into apposition to permit the redox chemistry. In the two monomers of transhydrogenase, there is a reciprocating, out-of-phase alternation of these conformations during turnover.     

Studies of translocation catalysis

There is a symbiotic relationship between the evolution of fundamental theory and the winning of experimentally-based knowledge. The impact of the General Chemiosmotic Theory on our understanding of the nature of membrane transport processes is described and discussed. The history of experimental studies on transport catalysed by ionophore antibiotics and the membrane proteins of mitochondria and bacteria are used to illustrate the evolution of knowledge and theory. Recent experimental approaches to understanding the lactose-H⁺ symport protein ofEscherichia coli and other sugar porters are described to show that the lack of experimental knowledge of the three-dimensional structures of the proteins currently limits the development of theories about their molecular mechanism of translocation catalysis.     

An autosome-Y translocation restudied

Summary It was proved by quinacrine fluorescence that a translocation of part of chromosome No. 2 had taken place on the short arm of the Y chromosome. A minimal loss of material at the breakage-reunion point apparently results in hypogonadism, as seen in this patient.Aided by contract No. 20.122 F.W.G.O., Belgium.Aspirant N.F.W.O., Belgium     

Endosomes and toxin translocation

In this review we discuss data obtained by our group regarding the entry of toxins, especially ricin, diphtheria toxin (DT) and Pseudomonas exotoxin A (PE) into animal cells. We studied the translocation process of these toxins using endosomes purified from lymphocytes. This process is rate-limiting for toxicity and enables these toxins to reach the cytosol where they will inactivate the protein synthesis system and kill the cell. We could show that each of these toxins uses a different strategy to cross the endosome membrane. Whereas ricin transmembrane transport only relies on cytosolic ATP hydrolysis, PE first requires exposure to the low endosomal pH (pH-6), presumably to insert into the endosome membrane, before being translocated via a process which also requires cytosolic ATP hydrolysis. DT translocation is directly triggered and energized by the endosome-cytosol pH gradient. Using conjugates with dihydrofolate reductase we could indirectly show that ricin and PE require unfolding for translocation. A deletion approach enabled to produce a more cytotoxic PE mutant by increasing its translocation activity.     

Protein translocation into mitochondria

The biogenesis of mitochondria requires the translocation of most mitochondrial proteins across two biological membranes. Mitochondrial preproteins are synthesized in the cytosol carrying targeting information, that is recognized by specific receptor proteins. The precursor polypeptides are transported across both mitochondrial membranes via three large integral membrane protein complexes forming specialized preprotein translocases. A soluble protein complex in the matrix provides the ATP-dependent translocation force, responsible for the movement and unfolding of the bulk polypeptide chain. After the removal of the targeting sequence, imported proteins fold into their native conformation with the help of chaperone proteins in the mitochondrial matrix.