Protein targeting plasma membrane proteins

NAPQl also acts as an electrophile, targeting the mitochondria in particular. This reactive metabolite forms covalent adducts with cellular molecules, particularly proteins with thiol groups. Other targets in addition to mitochondrial proteins include plasma membrane proteins involved in calcium homeostasis and adenine nucleotides (Dahm and Jones, 1996 Sturgill and Lambert, 1997). 

In intact cells insulin has been shown to stimulate receptor autophosphorylation. However, much smaller amounts of phosphotyrosine were found compared to that seen using purified, solubilized receptor preparations and, indeed, the predominant phosphorylation actually occurred on serine residues [69,71]. It has been suggested that there may be an insulin-stimulated, phosphoseryl-specific kinase which is loosely associated with the receptor and is activated by the receptor tyrosyl kinase [61]. This might account for the insulin-stimulated phosphoseryl kinase activity observed in both intact cells and when using crude, solubilized receptor preparations. Such an activity might also provide a mechanism for insulin s ability to enhance serine phosphorylation on target proteins in plasma membranes and elsewhere in the cell [25,78]. Nevertheless, (auto)-phosphorylation of the insulin receptor on tyrosine residues has been shown to occur immediately upon receptor occupancy by insulin and to precede any phosphorylation on serine residues [69]. Indeed, it remains to be seen as to whether the seryl phosphorylation occurs as a direct insulin-stimulation event or mediated by other kinases (cAMP/C-kinase) as a consequence of insulin-stimulated autophosphorylation on tyrosyl residues. 

Until recently, experiments that probe protein diffusion relied on fluorescent antibody-based probes, and thus were limited to plasma membrane proteins with extracellular epitopes. In contrast, the mechanisms that regulate diffusion of intracellular membrane proteins remained unexplored because of their inaccessibility to labeling. With the development of genetically encoded fluorescent probes, such studies have become tractable because proteins targeted to a particular organelle can be flu-orescently labeled directly with GEP. In addition, improved technology has now made it possible to monitor the movement of multiple types of proteins or lipids, tagged with different markers simultaneously. Some examples of the types of questions it is now possible to address include ... 

Neurotransmitters are released by exocytosis, a process in which neurotransmitter-filled synaptic vesicles fuse with the axonal membrane, releasing their contents into the synaptic cleft. The exocytosis of neurotransmitters from synaptic vesicles involves vesicle-targeting and fusion events similar to those that occur during the intracellular transport of secreted and plasma-membrane proteins (Chapter 17). Two features... 

Le Bivic, A., Real, F. X., and Rodriguez-Boulan, E. (1989) Vectorial targeting of apical and basolateral plasma membrane proteins in a human adenocarcinoma epithelial cell line. Proc. Natl. Acad. Sci. USA 86, 9313-9317. 

The use of fluorescent protein that is fused with a peptide motif for targeting proteins to plasma membrane such as the palmitoylation motif of GAP-43 [27] and the farnesylation motif (CAAX box) of K-ras [28] facilitates visualization of axons. However, it should be careful not to extremely overexpress the membrane-targeted version of fluorescent protein, because we have noticed that it sometimes causes morphological abnormality of axons. 

As an example of the interaction of a toxin that targets plasma membrane for its toxic action, features of staphylococcal a-toxin are briefly discussed to elaborate the relevant points. Staphylococcal a-toxin is hemolytic toxin that is secreted as a single water soluble 33 kDa protein (Jiang et al., 1991 Walker et al., 1992). 

PH domains consist of about 120 amino acid residues. They do not interact with other proteins, but associate with specific polyphosphoinositides. Consequently, PH domains appear to be important for localizing target proteins to the plasma membrane. Examples of PH domain-containing proteins include phospholipase C andpl20/RasGAP (Fig. 1). 

The biological actions of adrenaline and noradrenaline are mediated via nine different G-protein-coupled receptors, which are located in the plasma membrane of neuronal and nonneuronal target cells. These recqrtors are divided into two different groups, a-adrenergic receptors and P-adrenergic recqrtors (see P-adrenergic system). 

In the case of L-type Ca2+ channels, they also carry binding sites for Ca2+ antagonist drugs. The accessory a2-5, p, and y subunits stabilize Ca2+ channel function and support its targeting to the plasma membrane. Notably other proteins can associate with the channel complex allowing the formation of signaling complex important for channel targeting and modulation. 

Domains consisting of about 120 amino acid residues. Important for localizing target proteins to the plasma membrane. 

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