Chapter 15 - Intracellular Compartments and Protein Transport (Exam 4)

a typical eukaryotic cell can carry out…

thousands of different chemical reactions at any one time

the chemical reactions that a eukaryotic cell carries out are usually…

mutually incompatible

what needs to happen for a cell to operate effectively?

the different intracellular processes that occur simultaneously must be segregated

in order for the intracellular processes to be segregated, how can we isolate and organize different chemical reactions (two ways)?

1. large multicomponent complexes

2. different membrane-enclosed compartments

prokaryotic and eukaryotic cells aggregate different enzymes required to…

catalyze a particular sequence of reactions

the sequence of reactions that enzymes catalyze occur in…

large multicomponent complexes

give examples of large multicomponent complexes

synthesis of DNA, RNA and proteins

isolating and organizing different chemical reactions is most highly developed in…

eukaryotic cells

in eukaryotic cells, what is the purpose of different membrane-enclosed processes?

to confine different metabolic processes and the proteins required to perform them

membrane-enclosed compartments/organelles are also called…

intracellular compartments

in cells, intracellular compartments are packed together

what is an example of a cell that contains a basic set of organelles?

intestinal cell

main function of the nucleus

communicates with the cytosol via nuclear pores

what surrounds the nucleus?

nuclear envelope

nuclear envelope

double membrane that surrounds the nucleus

purpose of endoplasmic reticulum (ER)

major site for synthesis of new membranes

difference between rough ER and smooth ER

rough ER has ribosomes attached to its cytosolic surface

purpose of golgi apparatus

receives proteins and lipids from ER, modifies them, dispatches them to other places in the cells

lysosomes - what are they and what do they do?

• sacs of digestive enzymes

• degrade worn-out organelles, macromolecules, and particles taken into the cell by endocytosis

endocytosis

when a cell uses energy to engulf materials outside the cell through a membrane-bound vesicle

endosomes

compartments containing endocytosed materials

peroxisomes

• single membrane organelle

• contains enzymes used in a variety of oxidative reactions, detoxifies harmful substances

mitochondria carries out…

pyruvate oxidation, TCA cycle, oxidative phosphorylation

chloroplasts are responsible for…

photosynthesis

what process may have led to the evolution of nuclear membranes and the ER?

invagination of the plasma membrane

describe the process of invagination of the plasma membrane

• the plasma membrane essentially folds in, forming a 2 layered envelope completely surrounding the DNA

• other portions of the same membrane formed the ER, which ribosomes later attached to

origination of the mitochondria

mitochondria are thought to have originated when a prokaryote was engulfed by a larger eukaryotic cell

origination of choloroplasts

chloroplasts are thought to have originated when a eukaryotic cell engulfed a photosynthetic prokaryote

origination theory of mitochondria and chloroplasts helps explain…

why these organelles have two membranes and their own genomes

membrane-enclosed organelles import…

proteins

where does synthesis of almost all proteins begin?

the cytosol

synthesis of few mitochondrial and chloroplast proteins…

do not occur in the cytosol

sorting signal

directs the protein to the organelle in which it is required

what are the three mechanisms that membrane-enclosed organelles use to import proteins?

1. transport through nuclear pores

2. transport across membranes

3. transport by vesicles

transport through nuclear pores

proteins travel from the cytosol into the nucleus

transport across membranes

proteins travel from the cytosol into the ER, mitochondria, and chloroplast

transport by vesicles

proteins are loaded from the lumen (inside) of one compartment of the endomembrane system and discharged into the second compartment

signal sequences

direct proteins to the correct organelle

what do proteins destined for the ER possess?

an N-terminal sequence

if proteins lack the N-terminal sequence, they…

remain in the cytosol

the outer nuclear membrane is continuous with the…

ER

the double membrane of the nuclear envelope is penetrated by…

nuclear pores

through pores, traffic occurs in…

both directions

what are examples of materials needing to transport in and out of the nucleus through nuclear pores?

• newly made proteins

• RNA molecules

• ribosomal subunits

what does the nuclear pore complex do?

forms a gate through which molecules can enter or exit from the nucleus

the nuclear pore complex is a binding site for _________ and provides anchorage for the __________ _________

chromosomes, nuclear lamina

nuclear pore complexes are ______-________ passages

water-filled

why is the passage of large molecules not possible through the nuclear pore?

many proteins line the nuclear pore and contain extensive, unstructured regions

how are proteins bound for the nucleus transported through nuclear pores?

active transport (nuclear localization signal and nuclear transport receptors)

nuclear localization signal

the signal that directs a protein from the cytosol into the nucleus (located on the protein)

nuclear transport receptors

bind to the nuclear localization signal on newly synthesized proteins destined for the nucleus

what drives nuclear transport?

the energy supplied by GTP hydrolysis

what is the first step of nuclear transport (transporting a protein into the nucleus)?

the nuclear transport receptor picks up its cargo protein in the cytosol and enters the nucleus

what is the second step of nuclear transport?

in the nucleus, Ran-GTP binds to the nuclear transport receptor, which causes it to release the protein

what is the third step of nuclear transport?

the nuclear transport receptor - still carrying the Ran-GTP is transported back through the pore to the cytosol

what is the fourth step of nuclear transport?

in the cytosol, an accessory protein triggers Ran to hydrolyze its bound GTP to GDP

what is the last step of nuclear transport?

Ran-GDP falls off the nuclear transport receptor, which is then free to bind another cargo protein destined for the nucleus

what structure are proteins imported into the mitochondria as?

unfolded form

the precursor protein contains…

the mitochondrial signal sequence

the mitochondrial sequence is recognized by…

a receptor in the outer mitochondrial membrane

how do the complex of receptor and attached protein move in the membrane?

they diffuse laterally

the complex of receptor and attached protein diffuse laterally towards…

a contact site

what occurs at a contact site?

the protein is translocated across both the outer and inner membranes by a protein translocator

when is the signal sequence of the precursor protein cleaved off?

after it is translocated into the mitochondrion

how is the signal sequence of the precursor protein cleaved off?

by a signal peptidase

chaperone proteins

help to pull the protein across membranes

what is the most extensive membrane network in eukaryotic cells?

the ER

why is a common pool of ribosomes useful?

they synthesize proteins that stay in the cytosol and ER

where do membrane bound ribosomes attach?

the cytosolic side of the ER membrane

free ribosomes

unattached to any membrane

after each round of protein synthesis, what happens to ribosomal subunits?

they are released to rejoin the common pool in the cytosol

what helps direct a ribosome to the ER membrane?

an ER signal sequence and a SRP

SRP stands for…

signal recognition particle

what does the SRP bind to?

the exposed ER signal sequence and the ribosome

when the SRP binds to the ER signal sequence, what effect does this have on protein synthesis?

it slows down protein synthesis

the SRP-ribosome complex binds to…

an SRP receptor in the ER membrane

what happens when SRP is released?

the ribosome is passed to a translocation channel in the ER membrane

what does a translocation channel do?

inserts the polypeptide chain into the membrane and starts to transfer it across the lipid bilayer

what type of protein crosses the ER membrane and enters the lumen?

a soluble protein

purpose of translocation channel

• binds to the ER signal sequence

• actively transfers the rest of the polypeptide across the lipid bilayer as a loop

during the signal translocation process, what happens to the signal sequence?

it is cleaved from the growing protein

what cleaves the signal peptide from the protein translocator?

signal peptidase

after signal peptidase cleaves the signal, what two things happen?

1. the cleaved signal is ejected into the bilayer, where it is degraded

2. the translocated polypeptide (protein) is released as a soluble protein into the ER lumen

in order for a single-pass transmembrane protein to be integrated into the ER membrane, it needs initiation from…

an N-terminal ER signal sequence

what happens when the N-terminal ER signal sequence enters the translocation channel?

the channel discharges the protein sideways into the lipid bilayer

the N-terminal sequence is cleaved off and…

the transmembrane protein is now anchored in the membrane

what does a double-pass transmembrane protein need to integrate into the ER membrane?

internal start-transfer sequence

an internal ER signal sequence acts as…

a start-transfer signal and initiates the transfer of polypeptide chain

the internal start-transfer signal is recognized by…

an SRP

the SRP is responsible for…

bringing the ribosome to the ER membrane

when does the channel discharge both sequences into the membrane?

when a stop transfer sequence enters the translocation channel

what is specific about double-pass transmembrane proteins?

both the start-transfer sequence and stop-transfer sequence remain anchored in the membrane (they are not cleaved off)

read re-cap slide titled intracellular compartments and transport

make sure it makes sense

what do transport vesicles do?

carry soluble protein and membrane between cell compartments

vesicular transport that occurs in the endomembrane system between membrane-enclosed compartments is…

highly organized

outward secretory pathway, aka…

exocytosis

what is transported from where to where in the outward secretory pathway?

• synthesized proteins

• from the ER through the golgi to the plasma membrane

what is another way the outward secretory pathway may occur?

via early and late endosomes to lysosomes

inward endocytic pathway, aka…

endocytosis

what is transported where to where in the inward endocytic pathway?

• extracellular molecules are ingested in vesicles

• (vesicles come from the plasma membrane)

• delivered to early endosomes, then late endosomes, then lysosomes