Chapter 15: Intracellular Compartments and Protein Transport

In a typical human secretory cell, which of the following membranes has the largest surface area? (*hint: this membrane is folded up to form an extensive maze of interconnected spaces, and in some cases, this organelle can even compose about half of the total membrane present in the cell)

Rough ER

How do the interiors of the ER, Golgi apparatus, endoscopes, and lysosomes exchange contents with each other? (Hint* they carry solvable cargo proteins, as well as the proteins and lipids that are part of the vesicle membrane, from one organelle to another)

By small vesicles that bud off of one organelle and fuse with another

True or false. Membrane-bound ribosomes and free ribosomes are structurally and functionally identical; they differ only in the proteins they are making at a particular time. When a ribosome happens to be making a protein with an ER signal sequence, the signal sequence directs the ribosome to the ER membrane.

True

Which organelle cannot receive proteins directly from the cytosol? (*hint: proteins are delivered to this from the ER or other components of the endomembrane system)

Golgi apparatus

Vesicle budding is driven by which of the following?

Assembly of a protein coat

Botulism is a potentially fatal foodborne disease caused by the bacterium Clostridium botulinum. C. Botulinum produces different toxins, several of which are proteases that cleave neuronal SNARE proteins. What normal process is blocked by cleavage and inhibition of SNARE proteins?

Fusion of vesicle with target membranes

SNAREs help mediate vesicle membrane fusion. In the absence of vesicle fusion, vesicle-stored neurotransmitters cannot be released into synaptic clefts, leading to…

Paralysis

What distinguishes proteins destined for regulated secretion?

Their surface properties allow them to form aggregates that are packaged into secretory vesicles.

__________________ allows secretory proteins to be packaged into secretory vesicles are concentrations much higher than the concentration of the unaggregated protein in the Golgi lumen.

Selective aggregation

How are newly made lipids supplied to the plasma membrane? (*hint: the steady supply of lipids enables the plasma membrane to expand prior to cell division and replaces old lipids in nonproliferating cells)

Via the constitutive pathway of exocytosis

What happens to the final products of the digestion of macromolecules in the lysosome? (*hint: these molecule can then be reused by the cell or excreted)

They are transferred to the cytosol through transporters in the lysosomal membrane

Eukaryotic cells contain membrane-enclosed organelles, including a nucleus, an endoplasmic reticulum (ER), a Golgi apparatus, lysosomes, endoscopes, peroxisomes, mitochondria, and - in plant cells - chloroplasts. The ER, Golgi, peroxisomes, endoscopes, and lysosomes are all part of the…

Endomembrane system

Most organelle proteins are made in the cytosol and transported into the organelle where they function. __________________ 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.

Sorting signals

___________________ help direct the transport of nuclear proteins from the cytosol into the nucleus through nuclear pores, which penetrate the double membrane of the nuclear envelope. The proteins are transported into their fully folded conformations.

Nuclear localization signals

Most mitochondrial and chloroplast proteins are made in the cytosol and are then transported into the organelles by ____________________ in their membrane. The proteins are unfolded during the transport process.

Protein translocators

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 _________________ in the cytosol that recognizes an ER signal sequence on a growing polypeptide chain. The ribosomal-SRP complex binds to a receptor on the ER membrane, which passes the ribosome to a protein translocator that threads the growing polypeptide chain across the ER membrane.

Signal-recognition particle (SRP)

Water-soluable proteins destined for secretion or for the lumen of an organelle of the endomembrane system…

Pass completely into the ER lumen

Transmembrane proteins destined for either the membrane of these organelles or for the plasma membrane…

Remained anchored in the lipid bilayer by one or more membrane-spanning alpha helices.

In the ___________, proteins fold up, assemble with their protein partners, form disulfide bonds, and become decorated with oligosaccharide bonds.

ER lumen

Proteins that either fail to fold properly or fail to assemble with their normal partners are retained in the ER by _______________, which prevent their aggregation and help them fold

Chaperone proteins

Proteins that still fail to fold or assemble are transported to the cytosol to be…

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 and from the Golgi to other destinations is mediated by __________________ that continually bud off from one membrane and fuse with another, a process called vesicular transport

Transport vesicles

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 vesicle rapidly lose their protein coat, enabling then 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 (Rab, tethering, and SNARE proteins)

The Golgi receives newly made proteins from the ER; it modifies their oligosaccharides, sorts the proteins, and dispatches them from the ______________ to the plasma membrane, lysosomes (via endoscopes), or secretory vesicles.

Trans Golgi network

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

_____________________ 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

Specialized secretory cells

Cells ingest fluid, molecules, and something even particles by ________, in which regions of the plasma membrane invaginate and pinch off to form endocytic vesicles.

Endocytosis

Much of the material that is endocytosis is…

Delivered to endoscopes, which mature into lysosomes, where the material is degraded by hydrolysis enzymes.

Most of the components of the endocytic vesicle membrane are

Recycled in transport vesicles back to the plasma membrane for reuse