Connection of Protein Transport and Organelle Contact Sites …

The synthesis and release of secretory proteins in acinar cells of the rat pancreas


Similar to TnaC described above, the peptide SecM exists solely to stallthe ribosome synthesizing it. But unlike TnaC, which also requires thepresence of high levels of trytophan, SecM has an intrinsic stallingcapability. Stalling of the ribosome synthesizing SecM provides time fora downstream RNA helix on the same mRNA strand to unwind. Unwinding ofthis helix then allows for a new ribosome to bind and synthesize anew protein, SecA, a bacterial ATP-driven translocase that aids the passage ofnascent proteins across membranes in conjunction with SecY (see also ). When sufficient levels of SecA have been reached,SecA interacts with the SecM-stalled ribosome to pull on SecM, freeingit and allowing translation to resume (illustrated schematically inFig. 13). SecM, which serves no otherpurpose than to stall the ribosome, is released into the cell anddegraded.

Its primary function is that of serving as a site for enzyme action and storing various ions and enzymes for cellular functions.

which cell organelle is the site of protein synthesis ..

The nucleus is surrounded by the nuclear membrane
ribosome - small organelles composed of RNA-rich cytoplasmic granules that are sites of protein synthesis.
rough endoplasmic reticulum - (rough ER) a vast system of interconnected, membranous, infolded and convoluted sacks that are located in the cell's cytoplasm (the ER is continuous with the outer nuclear membrane).

Rough ER transports materials through the cell and produces proteins in sacks called cisternae (which are sent to the Golgi body, or inserted into the cell membrane).
smooth endoplasmic reticulum - (smooth ER) a vast system of interconnected, membranous, infolded and convoluted tubes that are located in the cell's cytoplasm (the ER is continuous with the outer nuclear membrane).

the sites of protein synthesis; ..

Due to great advances in the structural resolution of the ribosome, animpressive feat given its large size, the system is considered one ofthe hottest focal areas in molecular cell biology today. During theprocess of translation, the ribosome undergoes several conformationalchanges and binds to different factors that catalyze specificreactions. As detailed below, techniques to determine structure of theribosome can only image snapshots of the ribosome, often at medium tolow resolution. Atomic details of the interactions between the factorsand the ribosome, along with a dynamic description of the conformationalchanges of the ribosome itself, are crucial to understanding itsfunction.

Sep 22, 2014 sites of phospholipid synthesis in ..

Due to its sheer size and complexity, the ribosome presents anoutstanding challenge for traditional methods for high-resolutionstructure determination such as X-ray crystallography and nuclearmagnetic resonance spectroscopy. X-ray crystallographers have conquered thischallenge: today, the Protein Data Bank has several structures of entireribosomes from different laboratories. However, these structures remaindifficult to obtain for factor-bound ribosomes, which are key tounderstand the dynamics of translation.

free to attach at these sites to synthesize proteins and ..

Certain nascent peptide chains are able to regulate ribosome functionwhile they are still being synthesized, i.e., when they are still insidethe ribosomal exit tunnel. One of the classical examples is TnaC, aleader peptide of the tryptophanase operon in . At highconcentrations of tryptophan, TnaC stalls the ribosome, inhibitingtermination of its synthesis. Through an intricate gene regulatorymechanism, stalling ultimately leads to the expression of genesresponsible for degrading tryptophan.

The ribosomes are the site of protein synthesis.

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.