Biogenesis of MalF and the MalFGK(2) maltose transport complex in Escherichia coli requires YidC

The polytopic inner membrane protein MalF is a constituent of the MalFGK(2) maltose transport complex in Escherichia coli. We have studied the biogenesis of MalF using a combination of in vivo and in vitro approaches. MalF is targeted via the SRP pathway to the Sec/YidC insertion site. Despite close proximity of nascent MalF to YidC during insertion, YidC is not required for the insertion of MalF into the membrane. However, YidC is required for the stability of MalF and the formation of the MalFGK(2) maltose transport complex. Our data indicate that YidC supports the folding of MalF into a stable conformation before it is incorporated into the maltose transport complex.     

The lipid whisker model of the structure of oxidized cell membranes

An essential feature of the innate immune system is maintaining cellular homeostasis by identifying and removing senescent and apoptotic cells and modified lipoproteins. Identification is achieved through the recognition of molecular patterns, including structurally distinct oxidized phospholipids, on target cells by macrophage receptors. Both the structural nature of the molecular patterns recognized and their orientation within membranes has remained elusive. We recently described the membrane conformation of an endogenous oxidized phospholipid ligand for macrophage scavenger receptor CD36, where the truncated oxidized sn-2 fatty acid moiety protrudes into the aqueous phase, rendering it accessible for recognition. Herein we examine the generality of this conformational motif for peroxidized glycerophospholipids within membranes. Our data reveal that the addition of a polar oxygen atom on numerous peroxidized fatty acids reorients the acyl chain, whereby it no longer remains buried within the membrane interior but rather protrudes into the aqueous compartment. Moreover, we show that neither a conformational change in the head group relative to the membrane surface nor the presence of a polar head group is essential for CD36 recognition of free oxidized phospholipid ligands within membranes. Rather, our results suggest the following global phenomenon. As cellular membranes undergo lipid peroxidation, such as during senescence or apoptosis, previously hydrophobic portions of fatty acids will move from the interior of the lipid bilayer to the aqueous exterior. This enables physical contact between pattern recognition receptor and molecular pattern ligand. Cell membranes thus "grow whiskers" as phospholipids undergo peroxidation, and many of their oxidized fatty acids protrude at the surface.     

p-(Chloromercuri)benzenesulfonate binding by membrane proteins and the inhibition of water transport in human erythrocytes

The binding of [203Hg]-p-(chloromercuri)benzenesulfonate to the membrane proteins of human erythrocytes and erythrocyte ghosts was examined under conditions where binding to the bulk of membrane sulfhydryl groups was blocked by N-ethylmaleimide. Binding was essentially complete within 90 min when approximately 40 nmol was bound per milligram of membrane protein. This binding was correlated with the inhibition of water transport measured by an NMR technique. Maximal inhibition was observed with the binding of approximately 10 nmol of p-(chloromercuri)benzenesulfonate/mg of membrane protein. Under these conditions, both band 3 and band 4.5 bound 1 mol of inhibitor/mol of protein. In contrast to previous experiments, these results indicate that band 4.5 proteins as well as band 3 have to be considered as playing a role in water transport.     

NMR Studies of Backbone-Alkylated DNA: Duplex Stability,Absolute Stereochemistry,and Chemical Shift Anomalies of Prototypal Isopropyl Phosphotriester Modified Octanucleotides, (Rp,Rp)- and (Sp,Sp)-{d-[GGA(iPr)ATTCC]}2 and - {d- [GGAA(iPr)TTCC]}2

Abstract

The DNA octamer {d-[GGAATTCC]}₂ and four alkylated analogues, (Rp)-{d-[GGA(iPr)ATTCC]}₂, (Sp)-{d-[GGA(iPr)ATTCC]}₂, (Rp)-{d-[GGAA(iPr)TTCC]}₂, and (Sp)-{d-[GGAA(iPr)TTCC]}₂ have been examined using ¹H and ³¹P NMR spectroscopies. Duplex stability, as monitored by both NMR and optical measurements, is shown to be a function of both site and stereochemistry of the phosphotriester moiety. Chemical shift changes relative to the native octamer indicate that there are long-range perturbations in the isopropylated molecules. ¹H NMR is shown to be a general means by which stereochemistry at phosphorus can be determined.     

A model of soliton transport along microtubules

The uniform and jerklike microvesicle movement along microtubules as a result of soliton (a generated kink) formation on a filament is presented.