Lecture 24

Question 1

evolution of membrane-enclosed organelles

Answer 1

different organelles evolved in different ways
- nuclear membranes and endomembrane system (ER, Golgi, lysosomes, endoscopes, peroxisomes) may have risen through imagination of the plasma membrane
- the interiors of the endomembrane system communicate very intensely

Question 2

How are chloroplast and mitochondria thought to have originated

Answer 2

• mitochondria and chloroplast were through to have originated when a aerobic prokaryotes were engulfed by a pre-eukaryotic cells

Question 3

Where do most proteins begin synthesis

Answer 3

• almost all proteins begin their synthesis in the cytosol
• proteins destined to other organelles must be directed by a signal sequence which is dictate by an amino acid

Question 4

what are signal sequences

Answer 4

• typically 15-60 amino acids long
• function is dependent on properties of amino acids
• signal sequences are necessary and sufficient to direct proteins to a particular destination

Question 5

ways to transport proteins into organelle compartments

Answer 5

1. transport through nuclear pores
2. transport across membrane by using protein translocators
3. transport by vesicle

Question 6

Nuclear envelope has

Answer 6

• it contains and inner and outer membrane
• the outer membrane is contiguous with the ER
• it has nuclear pores which allow passage of molecules in and out of nucleus

Question 7

Nuclear pores

Answer 7

• composed of 30 proteins
• spans across the inner and outer membrane
• creates a fibril mesh net that fills the centre of the channel that prevent passage of molecules through it
• small, water soluble molecules can pass non selectively

Question 8

Transport through nuclear pore

Answer 8

• cytosolic proteins that are bound to the nucleus must contain nuclear localization signal (NLS)
• NLS is recognized by protein known as nuclear import receptors
• this will help direct the protein though nuclear pore by disrupting the interaction between the nuclear finer

Question 9

What drives nuclear transport

Answer 9

• energy from GTP

Question 10

Binding of Ran-GTP

Answer 10

• binding or Ran-GTP dissociates the imported protein from the receptor
• the receptor bound to Ran-GTP can be transported back into the cytosol where GTP is hydrolyzed to GDP
• the hydrolysis frees Ran-GTP from the receptor so it can bind to another NLS

Question 11

True or false: Both nuclear import and export depend on Ran-GTP

Answer 11

True

Question 12

Most mitochondria and chloroplast are synthesized in the ______ and transported across the membrane through __________

Answer 12

1. cytosol
2. protein translocators

Question 13

describe how mitochondria and chloroplast are transported

Answer 13

• proteins contain signal sequence a their N-terminus to allow their import
• proteins are unfolded as they are transported by a translocator
• signal sequence is removed after translocation
• chaperone proteins help proteins fold

Question 14

peroxisomes

Answer 14

• most peroxisomes proteins arrive from the cytosol
• they require translators to transport proteins across the membrane
• some proteins arrive from the ER
• the ER derived vesicles contain proteins that fuse with peroxisomes to help deliver their content

Question 15

if a protein is found in the lumen of the ER where could it end up

Answer 15

• the Golgi, lysosomes, endosomes and plasma membrane

Question 16

proteins in the endoplasmic reticulum

Answer 16

• protein in the cytosol bound to the ER have an ER signal sequence
• water soluble proteins in the lumen of the ER end up secreted or found in the lumen of an organelle in the endomembrane system
• transmembrane proteins end up in the membrane of the organelles of the endomembrane system or on the plasma membrane

Question 17

regions of the ER with ribosomes are called the _____

Answer 17

rough ER

Question 18

membrane bound ribosomes vs free ribosomes

Answer 18

• membrane bound ribosomes attach to the cytosolic side of the membrane while free ribosomes are not attached to any membrane
• they are both identical to one another

Question 19

polyribsomes are

Answer 19

• polyribosomes are many ribosomes attached to one mRNA molecule

Question 20

Signal recognition particle (SRP)

Answer 20

Directing ribosomes to the ER
- present in the cytosol
- binds to the ER signal sequence and the ribosomes

Question 21

SRP receptor

Answer 21

Directing ribosomes to the ER
- is embedded in the ER membrane
- binds to the SRP
- passes ribosome to a protein translator
- SRP is released
- protein synthesis occurs, passing the proton through the channel in the protein translocator

Question 22

Endoplasmic reticulum

Answer 22

• most proteins that enter the ER are threaded across the ER membrane before the polypeptide chain is fully synthesized

Question 23

Soluble proteins in the ER

Answer 23

• ER signal sequence causes the opening of the channel
• the signal sequence remains bound to the channel as the rest of the protein is threaded through
• once C-terminus has passed through, signal sequence is is removed by a signal peptidase on the luminal side of the ER and the protein is released into the lumen

Question 24

Transmembrane proteins in the ER

Answer 24

• for a single pass transmembrane protein, translocation is initiated by a start-transfer sequence
• translocation continues until a stop transfer sequence translocation
• translocators release growing polypeptide chain
• the stop transfer sequence form an alpha helix and remains embedded in the membrane
• start transfer is cleaved
• orientation of N and C will change from one side of the membrane to the other