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T1 - Synthesis and Turnover of Membrane Proteins in Rat Liver
AB - The assembly of the photosynthetic apparatus at the thylakoid begins with the targeting of proteins from their site of synthesis in the cytoplasm or stroma to the thylakoid membrane. Plastid-encoded proteins are targeted directly to the thylakoid during or after synthesis on plastid ribosomes. Nuclear-encoded proteins undergo a two-step targeting process requiring posttranslational import into the organelle from the cytoplasm and subsequent targeting to the thylakoid membrane. Recent investigations have revealed a single general import machinery at the envelope that mediates the direct transport of preproteins from the cytoplasm to the stroma. In contrast, at least four distinct pathways exist for the targeting of proteins to the thylakoid membrane. At least two of these systems are homologous to translocation systems that operate in bacteria and at the endoplasmic reticulum, indicating that elements of the targeting mechanisms have been conserved from the original prokaryotic endosymbiont.
After the accommodation of the tRNA brought by EF-Tu and successful peptidyl transfer, the ribosome is able to spontaneously undergo an intersubunit rotation (Fig. 1 (i) to (ii)). In this state the tRNAs assume a hybrid occupation relative to the ribosome: while the anticodon is still located in the original site on the small subunit, the other end of the tRNA is shifted to the next binding site on the large subunit generating so called A/P and P/E hybrid states. The P/E tRNA engages in interactions with the L1 stalk, a very dynamic part of the ribosome. These interactions are thought to stabilize the hybrid state. Interestingly, based on FRET experiments it seems that the initiator tRNA interacts weaker with the L1 stalk.
lipids from their site of synthesis to other ..
Once the structure determination of a membrane protein is finished, strong efforts are put into understanding function and mechanism by combined approaches using genetic methods, specific labelling and biophysical techniques. Such work includes site-specific mutagenesis and an enzymatic and structural characterization of the resulting variants. Various spectroscopic methods, like nuclear magnetic resonance (nmr) spectroscopy, Fourier Transform infrared (FTIR) spectroscopy and electron paramagnetic resonance spectroscopy are used to investigate structural changes connected to the membrane protein’s reaction cycle. These spectroscopic experiments are mainly done in collaborations with our colleagues from (nmr: Profs. , and , epr: , FTIR: ). For electrophysiological investigations we use the expertise of our colleagues from the institute’s . We measure electric currents and voltages accompanying the action of membrane proteins incorporated into lipid bilayers.
The photosynthetic reaction centre from the purple bacterium Rhodopseudomonas viridis (now called Blastochloris viridis), first atomic structure of a membrane protein. Hartmut Michel received the together with Johann Deisenhofer and Robert Huber for the structure determination.
The Rough ER Is The Site Of Synthesis Of Many Diff ..
Using MDFF and equilibrium MD we addressed the nature of the interactions between the L1 stalk and a tRNA in the hybrid P/E state. In particular, we compared the behavior of the initiator tRNAfMet versus the elongator tRNAPhe. Not only do the two tRNAs assume different conformations within the ribosome (Fig. 10), they interact differently with the L1 stalk. While for both tRNAs a peculiar stacking with the ribosomal RNA of the L1 stalk is observed, it is less pronounced for the initiator tRNA compared to an elongator tRNA (Fig. 10). Interestingly, the behavior of the tRNAs is strongly impacted by their respective modification patterns.
The structural basis for TnaC-mediated translational stalling wasaddressed by obtaining a 5.8-Å cryo-EM map of the ribosome stalled byTnaC and high concentrations of tryptophan (Fig. 8). The cryo-EM datashows that the nascent chain adopts a distinct conformation in the exittunnel. We applied MDFF to obtain an atomic model of the entire ribosomeand the stalling nascent chain (Fig. 8F). The model allowed us to mapthe contacts between TnaC and the exit tunnel, as well as proposepossible communication pathways that would lead to inactivation of thecatalytic center of the ribosome (the so-called peptidyltransferasecenter, or PTC). One of the main findings was that two criticalribosomal residues at the PTC adopt conformations that are incompatiblewith cohabitation by release factors, which catalyze termination ofprotein synthesis.
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The smooth ER is the site of synthesis of membrane lipids
N2 - The integral membrane protein bacteriorhodopsin, containing a fluorescent amino acid at a specific position, was synthesized in the presence of hydrated lipid films using an in vitro translation system expanded with a four-base codon/anticodon pair. Cell-sized liposomes with the labeled protein inserted into the liposome membranes were generated after the translation reaction. This study also demonstrated that this labeling method could be used to analyze the dynamic properties of membrane proteins in situ by fluorescence correlation spectroscopy.
Robust Chemical Synthesis of Membrane Proteins …
AB - The integral membrane protein bacteriorhodopsin, containing a fluorescent amino acid at a specific position, was synthesized in the presence of hydrated lipid films using an in vitro translation system expanded with a four-base codon/anticodon pair. Cell-sized liposomes with the labeled protein inserted into the liposome membranes were generated after the translation reaction. This study also demonstrated that this labeling method could be used to analyze the dynamic properties of membrane proteins in situ by fluorescence correlation spectroscopy.
Mechanisms of Protein Synthesis by the Ribosome
Proteolytic cleavage of proteins as part of processing. The later stages in processing of many secreted proteins involves proteolytic cleavage of a large to produce a smaller active protein. This typically occurs in secretory granules as they move away from the trans Golgi network. These cleavages are carried out by specific endoproteases (enzymes that cleave polypeptides at sites within the chain). This sort of cleavage is very common in the case of digestive enzymes.
Site of secretory and membrane protein synthesis ..
Membranes surround biological cells and divide the cells of higher organisms into various compartments. Biological membranes are composed of lipids, which form a bilayer, and of membrane proteins which are incorporated into the lipid bilayer. Lipid bilayers are impermeable to ions and to polar substances. As a consequence of this property ion gradients and electric voltages (“membrane potentials”) can be formed across membranes. Membrane proteins are required to enable the specific passage or transport of selected substances across membranes. Enabling passage and transport is therefore one of the most important functions of membrane proteins. Cells have to communicate, receive signals and to sense their environment. Many membrane proteins are sensors and receptors, the signal is often received at the outer face of the membrane and transduced across the membrane. Membrane proteins are central components of biological energy conversion. In photosynthesis and cellular respiration the transport of electrons and protons across membranes is the primary step of energy conversion. The resulting membrane potentials and ion gradients can be used to drive the synthesis of adenosine-5’-triphosphate (ATP), the uptake of nutrients, the export of waste products and proteins as well to drive the flagella motors. Finally, some membrane proteins are enzymes, in particular, when the substrates and/or products are hydrophobic.
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