In contrast to the situation in prokaryotes, RNA transcription and protein synthesis are separated in space and time in eukaryotes. Exchange of material between the nucleus and the cytoplasm is essential for the basic functioning of these cells and must be tightly controlled. RNA and ribosomal subunits that are assembled in the nucleus have to enter the cytoplasm where they are required for protein synthesis. On the other hand, proteins such as histones and transcription factors must enter the nucleus to carry out their functions. The nuclear pore complex mediates trafficking between the nucleus and the cytoplasm. The pore is made from a large number of proteins, and we refer to this type of structure as a multiprotein complex. Transport through the nuclear pore complex is mediated by signals and requires both energy and transporter proteins.
One of the functions of the smooth endoplasmic reticulum is to hold calcium ions ready for release into the cytosol when the cell is stimulated. A protein called calreticulin (short for calcium-binding protein of the endoplasmic reticulum) helps hold the calcium ions. Its primary structure is
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