Protein targeting membrane-spanning proteins

Many proteins will not yield crystals in initial crystal trials. Unfortunately, crystallization is still a trial-and-error procedure, with no real way of predicting success or failure. If you fail to get crystals in your initial trials, it need not be the end of your structural studies (although it could be ). Many of the targets for neuroscientists are going to be membrane-bound or membrane-spanning proteins, and these are notoriously difficult to crystallize. Techniques are continuously being developed and refined to improve our ability to crystallize difficult protein examples. 

Next, the authors turn to membrane proteins, the major functional constituents of biological membranes. The arrangement of proteins and lipids in membranes is described and the asymmetric, fluid nature of membranes is stressed. The important differentiation between integral and peripheral membrane proteins is discussed as well as the chemical forces that bind them to the membrane. The high-resolution analyses of the structures of selected membrane proteins are discussed, including structure prediction of membrane-spanning proteins. The chapter concludes with a discussion of internal membranes within eukaryotic cells and the mechanisms by which proteins are targeted to specific compartments within cells. 

NKCC is a heavily glycosylated protein with 12 putative membrane-spanning regions. Thirty percent of the sodium that is filtered by renal glomeruli is reabsorbed by Na-K-2C1 cotransport in the ascending limb of Henle in the nephron. Na-K-2C1 cotransport is a target of all loop diuretics. 

Ambros In the case of the LIN-12 phenomenon that I described, one of the experiments involved commitment in response to the activity of truncated LIN-12 protein lacking a membrane spanning domain. This form of LIN-12 likely acts in the nucleus. This constrains our thinking concerning the possible phosphorylation targets to nuclear components. 

Type I Proteins All type I transmembrane proteins possess an N-terminal signal sequence that targets them to the ER and an internal hydrophobic sequence that becomes the membrane-spanning a helix. The N-termlnal signal sequence on a nascent type I protein, like that of a secretory protein,... 

Translocation by the type III and type IV secretion systems, on the other hand, entails a single step. These systems consist of large protein complexes that span both membranes, allowing proteins to be translocated directly from the cytosol to the extracellular environment. The type III system is adapted not only for secreting proteins but also for injecting them into target cells, a very useful property for pathogenic bacteria. 

Thioacylation frequently dictates plasma membrane targeting of proteins lacking trans-membrane spans. In the case of p59 5 " targeting occurs directly, with the iV-myristoylated protein becoming thioacylated and plasma membrane associated rapidly upon completion of synthesis. In contrast, p56 appears to be thioacylated on intracellular membranes and arrive at the plasma membrane via vesicular transport (bound to the cytoplasmic face of secretory vesicles) (M. Bijlmakers, 1999). In yet another targeting variation, newly synthesized A-myristoylated G, a dually acylated trimeric G protein cx-subunit, associates with all cellular membranes but accumulates eventually at the plasma membrane the plasma membrane form is the only one that is both A-myristoylated and thioacylated. 

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