Membranes intracellular trafficking

Recent evidence indicates that the 5-HT transporter is subject to post-translational regulatory changes in much the same way as neurotransmitter receptors (Blakeley et al. 1998). Protein kinase A and protein kinase C (PKC), at least, are known to be involved in this process. Phosphorylation of the transporter by PKC reduces the Fmax for 5-HT uptake and leads to sequestration of the transporter into the cell, suggesting that this enzyme has a key role in its intracellular trafficking. Since this phosphorylation is reduced when substrates that are themselves transported across the membrane bind to the transporter (e.g. 5-HT and fi -amphetamine), it seems that the transport of 5-HT is itself linked with the phosphorylation process. Possibly, this process serves as a homeostatic mechanism which ensures that the supply of functional transporters matches the demand for transmitter uptake. By contrast, ligands that are not transported (e.g. cocaine and the selective serotonin reuptake inhibitors (SSRIs)) prevent the inhibition of phosphorylation by transported ligands. Thus, such inhibitors would reduce 5-HT uptake both by their direct inhibition of the transporter and by disinhibition of its phosphorylation (Ramamoorthy and Blakely 1999). 

Neurons constitute the most striking example of membrane polarization. A single neuron typically maintains thousands of discrete, functional microdomains, each with a distinctive protein complement, location and lifetime. Synaptic terminals are highly specialized for the vesicle cycling that underlies neurotransmitter release and neurotrophin uptake. The intracellular trafficking of a specialized type of transport vesicles in the presynaptic terminal, known as synaptic vesicles, underlies the ability of neurons to receive, process and transmit information. The axonal plasma membrane is specialized for transmission of the action potential, whereas the plasma... 

Figure 11.1 The intracellular trafficking pathway of plasmid DNA complexed by poly cationic lipid (lipoplex). Critical steps are indicated by numbers (1) endocytosis, sorting and recycling via vesicular compartments comprising the early (EE) and sorting endosomes (2) entrapment and degradation in the late-endosomes (LE) and lysosomes (3) destabilization of the endo-lysosomal membrane and release into the cytosol, (the precise location of this step is not known) (4) diffusion toward the nuclear pore complex (NPC) and degradation in the cytoplasm, and (5) nuclear translocation across the NPC.

Both TeNT and BoNTs bind the presynaptic membrane of a-motoneurons, but then TeNT follows a different intracellular trafficking route and this must be determined by yet unidentified specific receptor(s). 

All identified TLRs are type I transmembrane proteins, whose intracellular domains contain regions homologous to the intracellular domains of IL-1R and are referred to as TIR domains (Takeda et ah, 2003). These intracellular domains are able to trigger signalling pathways known to activate the nuclear factor kappa B (NF-kB) (Medzhitov et ah, 1998 O Neill, 2000), which in turn leads to the secretion of pro-inflammatory cytokines such as TNF-a, IL-6 and IL-8. The membrane distribution of TLRs as well as their intracellular trafficking has only now beginning... 

The NRG1 C-terminal intracellular domain (NRG1-ICD) is required for membrane insertion, intracellular trafficking, and surface expression for Type I NRG1 isoforms. In addition, the NRG1-ICD mediates novel signal transduction events, at least for the Type III isoforms. The importance of the NRG1-ICD has been substantiated from several lines of direct and indirect evidence. 

They are also important in intracellular trafficking and transport of retinoids. CRBP(II) interacts direcdy with the enterocyte membrane retinol transporter, and CRBP(I) with the cell surface RBP receptor, thus permitting direct uptake and accumulation of retinol from the intestinal lumen and circulation respectively. CRBP(l) is present in large amounts in cells that synthesize and... 

Eukaryotic cells encode numerous proteins that terminate with a CAAX motif ( 100 in yeast, and several hundred in mammalian cells), and many have been directly demonstrated to be prenylated [1,2,4,6]. Their CAAX modifications can contribute significant hydrophobicity and profoundly influence the biological properties of these proteins. For example, CAAX modifications have been shown to affect membrane association, protein-protein interactions, intracellular trafficking, and/or the stability of the proteins to which they are appended [3]. 

Cell membranes are two-dimensional fluids that exhibit a wide range of dynamic behaviors. Recent technical advances have enabled unprecedented views of membrane dynamics in living cells. In this technical review, we provide a brief overview of three well-studied examples of membrane dynamics lateral diffusion of proteins and lipids in the plane of the membrane, vesicular trafficking between intracellular compartments, and exchange of proteins on and off membranes. We then discuss experimental approaches to monitor membrane protein and lipid dynamics, and we place a special emphasis on the use of genetically encoded fluorescent probes and live cell-imaging techniques. 

Fukuda M. Lysosomal membrane glycoproteins. Structure, biosynthesis, and intracellular trafficking. J. Biol. Chem. 1991 266 21327-21330. 

Often, experimental studies of lipid systems are based on spectroscopic approaches, which in turn frequently employ probes for enhancement of sensitivity and resolution. For example, in NMR, hydrogen atoms of lipids are replaced with deuterium, and in fluorescence spectroscopy and imaging, native lipid molecules are replaced with lipids in which one of the hydrocarbon chains is linked covalently to a fluorescent marker such as pyrene or diphenylhexatriene. Fluorescent markers allow one to follow numerous cellular processes in real time, such as intracellular trafficking of molecules and formation of domains within a biomembrane, see Fig. 3. The downside is that the probes tend to perturb their environment and affect the thermodynamic state of the system. Experiments have shown, for example, that probes may change the main transition temperature of a lipid membrane, and that the dynamics of probes may deviate considerably from the dynamics of corresponding native molecules (see discussion in Reference 27). Therefore, we wish to pose several questions. What is the range of perturbations induced by the probe How significant are these perturbations actually ... 

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