Lecture 24 Flashcards

1
Q

evolution of membrane-enclosed organelles

A

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

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2
Q

How are chloroplast and mitochondria thought to have originated

A
  • mitochondria and chloroplast were through to have originated when a aerobic prokaryotes were engulfed by a pre-eukaryotic cells
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3
Q

Where do most proteins begin synthesis

A
  • 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
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4
Q

what are signal sequences

A
  • 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
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5
Q

ways to transport proteins into organelle compartments

A
  1. transport through nuclear pores
  2. transport across membrane by using protein translocators
  3. transport by vesicle
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6
Q

Nuclear envelope has

A
  • 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
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7
Q

Nuclear pores

A
  • 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
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8
Q

Transport through nuclear pore

A
  • 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
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9
Q

What drives nuclear transport

A
  • energy from GTP
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10
Q

Binding of Ran-GTP

A
  • 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
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11
Q

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

A

True

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12
Q

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

A
  1. cytosol
  2. protein translocators
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13
Q

describe how mitochondria and chloroplast are transported

A
  • 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
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14
Q

peroxisomes

A
  • 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
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15
Q

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

A
  • the Golgi, lysosomes, endosomes and plasma membrane
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16
Q

proteins in the endoplasmic reticulum

A
  • 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
17
Q

regions of the ER with ribosomes are called the _____

A

rough ER

18
Q

membrane bound ribosomes vs free ribosomes

A
  • 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
19
Q

polyribsomes are

A
  • polyribosomes are many ribosomes attached to one mRNA molecule
20
Q

Signal recognition particle (SRP)

A

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

21
Q

SRP receptor

A

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

22
Q

Endoplasmic reticulum

A
  • most proteins that enter the ER are threaded across the ER membrane before the polypeptide chain is fully synthesized
23
Q

Soluble proteins in the ER

A
  • 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
24
Q

Transmembrane proteins in the ER

A
  • 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