Essential Cell Biology 5th edition

Essential Concepts
529
• 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 of the
nuclear envelope. The proteins are transported in their fully folded
conformation.
• Most mitochondrial and chloroplast proteins are made in the cytosol
and are then transported into the organelles by protein translocators
in their membranes. The proteins are unfolded during the transport
process.
• The ER makes most of the cell’s lipids and many of its proteins. The
proteins are made by ribosomes that are directed to the ER by a
signal-recognition particle (SRP) in the cytosol that recognizes an ER
signal sequence on the growing polypeptide chain. The ribosome–
SRP complex binds to a receptor on the ER membrane, which passes
the ribosome to a protein translocator that threads the growing polypeptide
across the ER membrane.
• Water-soluble proteins destined for secretion or for the lumen of an
organelle of the endomembrane system pass completely into the
ER lumen, while transmembrane proteins destined for either the
membrane of these organelles or for the plasma membrane remain
anchored in the lipid bilayer by one or more membrane-spanning
α helices.
• In the ER lumen, proteins fold up, assemble with their protein
partners, form disulfide bonds, and become decorated with oligosaccharide
chains.
• Exit from the ER is an important quality-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, which prevent
their aggregation and help them fold; proteins that still fail to fold or
assemble are transported to the cytosol, where they are degraded.
• Excessive accumulation of misfolded proteins triggers an unfolded
protein response that expands the ER, increases its capacity to fold
new proteins properly, and reduces protein synthesis.
• Protein transport from the ER to the Golgi apparatus 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 vesicular transport.
• Budding transport vesicles have distinctive coat proteins on their
cytosolic surface; the assembly of the coat helps drive both the budding
process and the incorporation of cargo receptors, with their
bound cargo molecules, into the forming vesicle.
• Coated vesicles rapidly lose their protein coat, enabling them to dock
and then fuse with a particular target membrane; docking and fusion
are mediated by proteins on the surface of the vesicle and target
membrane, including Rab, tethering, and SNARE proteins.
• The Golgi apparatus receives newly made proteins from the ER; it
modifies their oligosaccharides, sorts the proteins, and dispatches
them from the trans Golgi network to the plasma membrane,
lysosomes (via endosomes), or secretory vesicles.
• In all eukaryotic cells, transport vesicles continually bud from the
trans Golgi network and fuse with the plasma membrane; this process
of constitutive exocytosis delivers proteins to the cell surface
for secretion and incorporates lipids and proteins into the plasma
membrane.
• Specialized secretory cells also have a regulated exocytosis pathway,
in which molecules concentrated and stored in secretory vesicles
are released from the cell by exocytosis when the cell is signaled to
secrete.