<P> The surface of the rough endoplasmic reticulum (often abbreviated RER or Rough ER) (also called granular endoplasmic reticulum) is studded with protein - manufacturing ribosomes giving it a "rough" appearance (hence its name). The binding site of the ribosome on the rough endoplasmic reticulum is the translocon . However, the ribosomes are not a stable part of this organelle's structure as they are constantly being bound and released from the membrane . A ribosome only binds to the RER once a specific protein - nucleic acid complex forms in the cytosol . This special complex forms when a free ribosome begins translating the mRNA of a protein destined for the secretory pathway . The first 5--30 amino acids polymerized encode a signal peptide, a molecular message that is recognized and bound by a signal recognition particle (SRP). Translation pauses and the ribosome complex binds to the RER translocon where translation continues with the nascent (new) protein forming into the RER lumen and / or membrane . The protein is processed in the ER lumen by an enzyme (a signal peptidase), which removes the signal peptide . Ribosomes at this point may be released back into the cytosol; however, non-translating ribosomes are also known to stay associated with translocons . </P> <P> The membrane of the rough endoplasmic reticulum forms large double membrane sheets that are located near, and continuous with, the outer layer of the nuclear envelope . The double membrane sheets are stacked and connected through several right or left - handed helical ramps, the so - called Terasaki ramps, giving rise to a structure resembling a multi-storey car park . Although there is no continuous membrane between the endoplasmic reticulum and the Golgi apparatus, membrane - bound transport vesicles shuttle proteins between these two compartments . Vesicles are surrounded by coating proteins called COPI and COPII . COPII targets vesicles to the Golgi apparatus and COPI marks them to be brought back to the rough endoplasmic reticulum . The rough endoplasmic reticulum works in concert with the Golgi complex to target new proteins to their proper destinations . A second method of transport out of the endoplasmic reticulum involves areas called membrane contact sites, where the membranes of the endoplasmic reticulum and other organelles are held closely together, allowing the transfer of lipids and other small molecules . </P> <P> The rough endoplasmic reticulum is key in multiple functions: </P> <Ul> <Li> Manufacture of lysosomal enzymes with a mannose - 6 - phosphate marker added in the cis - Golgi network . </Li> <Li> Manufacture of secreted proteins, either secreted constitutively with no tag or secreted in a regulatory manner involving clathrin and paired basic amino acids in the signal peptide . </Li> <Li> Integral membrane proteins that stay embedded in the membrane as vesicles exit and bind to new membranes . Rab proteins are key in targeting the membrane; SNAP and SNARE proteins are key in the fusion event . </Li> <Li> Initial glycosylation as assembly continues . This is N - linked (O - linking occurs in the Golgi). <Ul> <Li> N - linked glycosylation: If the protein is properly folded, Oligosaccharyltransferase recognizes the AA sequence N X S or N X T (with the S / T residue phosphorylated) and adds a 14 - sugar backbone (2 - N - acetylglucosamine, 9 - branching mannose, and 3 - glucose at the end) to the side - chain nitrogen of Asn . </Li> </Ul> </Li> </Ul>

Describe the structure and function of ribosomes endoplasmic reticula and vacuole