Messenger RNA exported from the nucleus binds to a ribosome in the cytosol fluid portion of cytosol, which then proceeds to translate the genetic message into a protein. a ce^ not including the Some proteins, with their ribosomes, remain free in the cytosol throughout translation, but others do not. Those that do not remain free carry a special sequence of amino acids at their leading end, called a signal peptide. This sequence directs the growing protein with its ribosome to the surface of the endoplasmic reticulum (ER), the most extensive organelle in the cell. Here, the ribosome attaches and extrudes the growing protein into the interior, or lumen, of the ER. Attachment of numerous ribosomes gives portions of the ER a rough appearance under the electron microscope. The ER also synthesizes most of the lipids used in the cell's many membranes. Lipid-synthesizing ER does not have ribosomes attached, and so appears smooth.
Many of the proteins entering the ER lumen are destined for other compartments in the cell, and contain organelle-specific targeting sequences that code organelles
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