Chap 15

Return to Essential Concepts


  • Eukaryotic cells contain many membrane-enclosed organelles, including a nucleus, an ER, a Golgi, lysosomes, mitochondria, chloroplasts (plant cells) and peroxisome.
  • Most organelle proteins are made in the cytosol and transported into the organelle where they function. Sorting signals in the amino acid sequence guide the proteins to the correct organelle; proteins that function in the cytosol have no such signals and remain where they are made.
  • Nuclear proteins contain nuclear localization signals that help direct their active transport from the cytosol into the nucleus through nuclear pores, which penetrate the double-membrane nuclear envelope. Proteins enter the nucleus without being unfolded.
  • Most mitochondrial and chloroplast proteins are made in the cytosol and are then actively transported into the organelles by protein translocators in their membranes. Proteins must be unfolded to allow them to snake through the translocators in the chloroplast or mitochondrial membrane.
  • The ER is the membrane factory of the cell; it makes most of the cell's lipids and many of its proteins. The proteins are made of ribosomes bound to the surface of the rough ER.
  • Ribosomes in the cytosol are directed to the ER if the protein they are making has an ER signal sequence., which is recognized by a signal-recognition particle (SRP) in the cytosol; the binding of the ribosome-SRP complex to the receptor on the ER membrane initiates the translocation process that threads the growing polypeptide across the ER membrane through a translocation cell.
  • Soluble proteins destined for secretion or the lumen of an organelle pass completely into the ER lumen, while transmembrane proteins destined for the ER membrane or for other cell membranes remain anchored in the lipid bilayer by one or more membrane-spanning alpha-helices.
  • In the ER lumen, proteins fold up, assemble with other proteins, form disulfide bonds and become decorated with oligosaccharide chains.
  • Exit from the ER is an important quanlity-control step; proteins that either fail to fold properly or fail to assemble with their normal partners are retained in the ER by chaperone proteins and are eventually degraded.
  • As accumulation of misfolded proteins triggers a response that expands the size of the ER, thus increasing its capacity to fold new proteins properly.
  • Protein transport from the ER to the Golgi and from the Golgi apparatus to other destinations is mediated by transport vesicles that continually bud off from one membrane and fuse with another, a process called vesicle transport.
  • Budding transport vesicles have distinctive coat proteins on their cytosolic surface; the assembly of the coat drives the budding process, and the coat proteins help incorporate receptors with their bound cargo molecules into the forming vesicle.
  • Coated vesicles lose their protein coat soon after pinching off, enabling them to dock and then fuse with a particular target membrane; docking and fusion are mediated by proteins on the vesicle and on the target membranes, including Rab proteins and SNAREs.
  • The Golgi apparatus receives newly made proteins from the ER; it modifies their oligosaccharides, sorts the proteins, and dispatches from the trans Golgi network to the plasma membrane, lysosomes or the secretory vesicles.
  • In all eukaryotic cells, transport vesicles continually bud from the trans Golgi network and fuse with the plasma membrane, a process called constitutive exocytosis; the process delivers plasma membrane lipids and proteins to the cell surface and also releases molecules from the cell in the process of secretion.
  • Specialized secretory cells also have a regulated exocytosis pathway, where molecules stored in secretory vesicles are released from the cell by exocytosis when the cell is signaled to secrete.
  • Cells ingest fluid, molecules, and sometimes even particles, by endocytosis, in which regions of plasma membrane invaginate and pinch off to form endocytic vesicles.
  • Much of the material that is endocytosed is delivered to the endosomes and then to the lysosomes, where it is degraded by hydrolytic enzymes; most of the components of the endocytic vesicle membrane, however, are recycled in transport vesicles back to the plasma membrane for reuse.

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