F7, F8 og F9, Intracellulær proteinsortering

Question 1

Show how a cell from the lining of the intestine contains the basic set of membrane-enclosed organelles found in most animal cells

Answer 1

Question 2

Show how A common pool of ribosomes is used to synthesice all the proteins encoded by the nuclear genome

Answer 2

Question 3

Show how a double-pass transmemrane protein has an internal ER signal sequence

Answer 3

Question 4

Show that a lysosome contains a large variety of hydrolytic enzymes, which are only active under acidic conditions

Answer 4

Question 5

Show that a single-pass transmembrane protein is retained in the lipid bilayer.

Answer 5

Question 6

Show that a soluble protein crosses the ER membrane and enter the lumen

Answer 6

Question 7

show that accumulation of misfolded proteins in the ER lumen triggers an unfolded protein response

Answer 7

Question 8

Show that An ER signal sequence and an SRP direct a ribosome to the ER membrane

Answer 8

Question 9

Show that clathrin-coated vesicles transport selected cargo molecules

Answer 9

Question 10

Show how energy supplied by CP hydrolysis drives nuclear transport

Answer 10

Question 11

Show how following vesicle docking, SNARE proteins can catalyze the fusion of the vesicle and target membranes

Answer 11

Question 12

Show how in secretory cells, the regulated and constitutive pathways of exocytosis diverge in the trans Golgi network

Answer 12

Question 13

Show how LDL enters cells via receptor-mediated endocytosis

Answer 13

Question 14

Show how Many proteins are glycosylated on asparagines in the ER

Answer 14

Question 15

Show how materials destined for degradation in lysosomes follow different pathways to the lysosome

Answer 15

Question 16

Show how membrane-enclosed organelles import proteins by one of three mechanisms

Answer 16

Question 17

Show how mitochondria are thought to have originated when an aerobic bacterium was engulfed by a larger anaerobic aukaryotic cell

Answer 17

Question 18

Show how mitochondrial precursor proteins are unfolded during import

Answer 18

Question 19

Show how Nuclear membranes and the ER may have evolved through invagination of the plasma membrane

Answer 19

Question 20

Essential concepts

Answer 20

• Eukaryotic cells contain many membrane-enclosed organelles, including a nucleus, an endoplasmic reticulum (ER), a Golgi apparatus, lysosomes, endosomes, mitochondria, chloroplasts (in plant

cells), and peroxisomes. The ER, Golgi apparatus, peroxisomes, endosomes, 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. 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 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 heir 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.

• 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 endosomes, which mature into lysosomes, in which the material 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.

Question 21

Show how prospective nuclear proteins are imported from the cytosol through nuclear pores

Answer 21

Question 22

Show how rab proteins, tethering proteins, and SNAREs help direct transport besivles to their target membranes

Answer 22

Question 23

Show how secretory vesicles store and release concentrated proteins

Answer 23

Question 24

Explain the different methods to show how a protein is transported

Answer 24

Question 25

Show how signal sequences direct proteins to the correct destination

Answer 25

Question 26

Make a table over some type of coated vesciles

Answer 26

Question 27

Make a table over some typical signal sequence

Answer 27

Question 28

Show how temperature-sensitive mutants have been used to dissect the protein secretory pathway in yeast

Answer 28

Question 29

Show how the fate of receptor proteins following their endocytosis depends on the type of receptor.

Answer 29

Question 30

Show how the golgi apparatus consists of a stack of flattened membrane-enclosed sacs

Answer 30

Question 31

Show how the main functions of membrane-enclosed organelles of a eukaryotic cell.

Answer 31

Question 32

Show how the nuclear pore complex forms a gate through which selected macromolecules and larger complexes enter or exit the nucleus

Answer 32

Question 33

Show how the outer nuclear membrane is continous with the ER membrane

Answer 33

Question 34

Make a table over the relative volumes and numbers of the major membrane-enclosed organelles in a liver cell

Answer 34

Question 35

Show how transport vesicles bud from one membrane and fuse with another

Answer 35

Question 36

Show how vesicular transport allows materiale to exit or enter the cell.

Answer 36

Question 37

Show how viruses can enter cells vi receptor-mediated endocytosis

Answer 37