<P> The nuclear pores are highly efficient at selectively allowing the passage of materials to and from the nucleus, because the nuclear envelope has a considerable amount of traffic . RNA and ribosomal subunits must be continually transferred from the nucleus to the cytoplasm . Histones, gene regulatory proteins, DNA and RNA polymerases, and other substances essential for nuclear activities must be imported from the cytoplasm . The nuclear envelope of a typical mammalian cell contains 3000--4000 pore complexes . If the cell is synthesizing DNA each pore complex needs to transport about 100 histone molecules per minute . If the cell is growing rapidly, each complex also needs to transport about 6 newly assembled large and small ribosomal subunits per minute from the nucleus to the cytosol, where they are used to synthesize proteins . </P> <P> The endoplasmic reticulum (ER) is a membranous synthesis and transport organelle that is an extension of the nuclear envelope . More than half the total membrane in eukaryotic cells is accounted for by the ER . The ER is made up of flattened sacs and branching tubules that are thought to interconnect, so that the ER membrane forms a continuous sheet enclosing a single internal space . This highly convoluted space is called the ER lumen and is also referred to as the ER cisternal space . The lumen takes up about ten percent of the entire cell volume . The endoplasmic reticulum membrane allows molecules to be selectively transferred between the lumen and the cytoplasm, and since it is connected to the nuclear envelope, it provides a channel between the nucleus and the cytoplasm . </P> <P> The ER has a central role in producing, processing, and transporting biochemical compounds for use inside and outside of the cell . Its membrane is the site of production of all the transmembrane proteins and lipids for most of the cell's organelles, including the ER itself, the Golgi apparatus, lysosomes, endosomes, mitochondria, peroxisomes, secretory vesicles, and the plasma membrane . Furthermore, almost all of the proteins that will exit the cell, plus those destined for the lumen of the ER, Golgi apparatus, or lysosomes, are originally delivered to the ER lumen . Consequently, many of the proteins found in the cisternal space of the endoplasmic reticulum lumen are there only temporarily as they pass on their way to other locations . Other proteins, however, constantly remain in the lumen and are known as endoplasmic reticulum resident proteins . These special proteins contain a specialized retention signal made up of a specific sequence of amino acids that enables them to be retained by the organelle . An example of an important endoplasmic reticulum resident protein is the chaperon protein known as BiP which identifies other proteins that have been improperly built or processed and keeps them from being sent to their final destinations . </P> <P> The ER is involved in cotranslational sorting of proteins . A polypeptide which contains an ER signal sequence is recognised by a signal recognition protein which halts the production of the protein . the SRP transports the polypeptide to the ER membrane where its released in through a membrane pore and translation resumes . </P>

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