Transmembrane movement

There have been two main strategies to study the transport of proteins into mitochondria and chloroplasts  

What have these two approaches told us about the transport step  

The difference between the energy requirement of chloroplasts and mitochondria for import is unclear. It may be the result of the difference in the nature of the membranes that are being crossed (a membrane with electron transport and oxidative phosphorylation activities in mitochondria and a membrane which mainly functions in solute translocation in chloroplasts). If this is the explanation, one may predict that plastocyanin, which is located in the cistemae of the thylakoids [91], would be imported in two steps the first dependent on ATP and the final transthylakoid movement dependent on a membrane potential. 

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All of the transport systems examined thus far are relatively large proteins. Several small molecule toxins produced by microorganisms facilitate ion transport across membranes. Due to their relative simplicity, these molecules, the lonophore antibiotics, represent paradigms of the mobile carrier and pore or charmel models for membrane transport. Mobile carriers are molecules that form complexes with particular ions and diffuse freely across a lipid membrane (Figure 10.38). Pores or channels, on the other hand, adopt a fixed orientation in a membrane, creating a hole that permits the transmembrane movement of ions. These pores or channels may be formed from monomeric or (more often) multimeric structures in the membrane. 

Structural models for voltage-dependent gating of ion channels must identify the voltage-sensors or gating charges (Fig. 6-5A) within the channel structure and suggest a plausible mechanism for transmembrane movement... 

Annexin V is a human placental anticoagulant protein of molecular weight 35kDa that binds to membranes and lipid bilayers containing phosphatidylserine in the presence of free calcium. Annexin V binding to cell surfaces said to result from transmembrane movement of... 

Methods of assessing transmembrane movement of sphingolipids have been reviewed by Sillence et al. (2000). 

Application of part of the classical Hantzsch pyridine synthesis leads to nifedipine (87) (81 AG(E)762, 68SAP6801482), a calcium antagonist useful in the treatment of angina. The pharmacology of a chemically related drug, nisoldipine (88), has recently been studied (80AF2144). Both compounds inhibit the transmembrane movement of calcium into activated smooth and cardiac muscle. Nisoldipine, however, is characterized by a high potency and uniqueness of action and may well prove to be of considerable therapeutic value. 

Two other types of specialized transport mechanisms, pinocytosis and phagocytosis, may also account for the transmembrane movement of some macromolecules (2). In these complex processes, the cell engulfs a droplet of extracellular fluid or a particle of solid material such as a bacterium. The droplet or particle is completely surrounded by a portion of the cell membrane and the resulting vesicle becomes detached and moves into the cell cytoplasm. 

Bishop, W.R. Bell, R.M. (1988) Assembly of phospholipids into cellular membranes biosynthesis, transmembrane movement and intracellular translocation. Ararat. Rev. Cell Biol. 4, 579-610. Advanced review of the enzymology and cell biology of phospholipid synthesis and targeting. 

Since Ki is expressed as a ratio, any consistent measure of composition in the membrane and external phases may be used in Equation 7.2. When K> 1, the membrane acts as a concentrator that attracts component i from the external phase and makes it available at the membrane surface for transmembrane movement. Intermolecular forces of solvation and mixing that are responsible for the partitioning process may be entropic as well as enthalpic in origin. The balance of these forces acting between the membrane and external phase can cause either a higher or lower concentration of a given solute inside the membrane relative to the external phase. If the tendency to enter the membrane is negligible, the partition coefficient approaches zero, that is, Kj —> 0. 

Eytan GD, Regev R, Oren G, et al. Efficiency of P-glycoprotein-mediated exclusion of rhodamine dyes from multidrug-resistant cells is determined by their passive transmembrane movement rate. Eur J Biochem 1997 248(1) 104—112. 

Because lipid solubility is so important for transmembrane movement of a drug, attempts have been made over the years to assess this characteristic as a predictor of drug activity. Perhaps the most useful method employs a simple relationship referred... 

Sodium and potassium cations are often encountered in the same biological environment and the transmembrane movements of both are required as part of an enzymatic pathway as in Na+, K+-ATPase. Under these circumstances it is essential that cation-specific channels are formed. What features of the channels contribute to the selectivity Earlier the preferred geometries of Na+and K+, sixfold octahedral and eightfold cubic respectively, were proposed as the main discriminatory factors. A computational analysis by Dudev and Lim [35] has considered the effect of coordinated water, number of available coordination sites in the channel walls, and the dipoles of the coordinating groups. The researchers investigated cation complexes with valinomycin and the protein KcsA, both K+-selective, and compared these with a non-selective NaK channel. 

Transmembrane Movement of Reducing Equivalents Under aerobic conditions, extramitochon-drial NADH must be oxidized by the mitochondrial electron-transfer chain. Consider a preparation of rat hepatocytes containing mitochondria and all the cytosolic enzymes. If [4-3H]NADH is introduced, radioactivity soon appears in the mitochondrial matrix. However, if [7-14C]NADH is introduced, no radioactivity appears in the matrix. What do these observations reveal about the oxidation of extramito-chondrial NADH by the electron-transfer chain ... 

Sillence, D.J., Raggers, RJ. and van Meer, G., 2000, Assays for transmembrane movement of sphingolipids. Methods Enzymol. 312 562-579. 

Helenius J, Aebi M. Transmembrane movement of dolichol linked carbohydrates during iV-glycoprotein biosynthesis in the endoplasmic reticulum. Semin. Cell. Dev. Biol. 2002 13 171-178. 

Black PN, DiRusso CC. Transmembrane movement of exogenous long-chain fatty acids proteins, enzymes, and vectorial esterification. Microbiol. Mol. Biol. Rev. 2003 67 454-472. 

Lange Y, Dolde J, Steck TL. The rate of transmembrane movement of cholesterol in the human erythrocyte. J. Biol. Chem. 25. 1981 256 5321-5323. 

A. Zachowski and P.F. Devaux. 1990. Transmembrane movements of lipids Experientia 46 644-656. (PubMed)... 

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