cell bio ch 12

What are organelles?

Functionally distinct membrane-enclosed compartments inside cells; each with its own enzymes and specialized molecules (~half of cell volume)

What is the function of the nucleus?

Contains genome (except mitochondrial/chloroplast DNA); site of DNA and RNA synthesis

What is the cytoplasm?

Cytosol and organelles suspended in it; site of protein synthesis/degradation and intermediary metabolism

What does the endoplasmic reticulum do?

Half of total membrane area; rough ER has ribosomes and makes proteins; smooth ER makes lipids and stores Ca2+

What do ribosomes do?

Synthesize soluble and membrane proteins for secretion or organelles; not membrane-bound

What is the Golgi apparatus?

modifies proteins, receives proteins and lipids from the ER and dispatches them to various locations, made of stacks of cisternae

What do mitochondria and chloroplasts do?

Generate ATP; chloroplasts also store food or pigments

What do lysosomes do?

Contain digestive enzymes to degrade macromolecules and organelles

What do endosomes do?

Carry and sort material brought into cell before lysosomal digestion

What are peroxisomes?

Small vesicles with oxidative enzymes for various reactions

What determines localization of organelles?

Interactions with cytoskeleton; microtubules localize ER and Golgi

What is the lumen?

Interior of organelles; topologically equivalent to outside of cell; communicates via vesicles

What are the four organelle families?

1. Nucleus and cytosol (communicate through NPCs, topologically continuous) - Secretory and endocytic pathway organelles - ER, Golgi, endosomes, lysosomes, transport vesicles, peroxisomes - Mitochondria - Plastids (plants only)

What are sorting signals?

Amino acid sequences directing proteins to organelles; lack of signal = cytosol residency

What are the four types of protein transport?

Gated transport, protein translocation, vesicular transport, engulfment

What is gated transport?

Protein/RNA movement between cytosol & nucleus via nuclear pore complexes (NPCs)

What is protein translocation?

Transmembrane translocators move unfolded proteins into topologically distinct spaces

What is vesicular transport?

Vesicles ferry proteins between equivalent compartments (e.g. ER → Golgi)

What are sorting receptors?

Recognize sorting signals; catalytically reused for multiple transport cycles

What are signal sequences?

Amino acid stretches (often N-terminus) that direct proteins; may be removed or retained

What are signal patches?

3D arrangements of amino acids forming sorting signals on protein surfaces

Give examples of signal sequences.

To ER: N-terminal; ER resident: C-terminal return; mitochondria: + and hydrophobic; peroxisome: C-terminal tripeptide

What is the nuclear envelope?

Double membrane surrounding DNA; continuous with ER; has nuclear pore complexes

What does the inner nuclear membrane do?

Binds chromosomes and nuclear lamina; structural support

What does the outer nuclear membrane do?

Continuous with ER and studded with ribosomes, protein synthesis → proteins transported into perinuclear space between inner and outer membrane, which is continuous with ER membrane

What are nuclear pore complexes (NPCs)?

Large protein structures (~30 nucleoporins) allowing selective bidirectional transport

What is the function of FG repeats?

Bind nuclear import receptors for cargo transport through NPCs

What are nuclear localization signals (NLSs)?

Amino acid sequences rich in lysine/arginine that direct nuclear import

What do importins do?

Nuclear import receptors that bind NLSs and ferry cargo through NPCs

What do exportins do?

Bind nuclear export signals to export proteins from nucleus

What is Ran GTPase?

A GTPase controlling directionality of nuclear transport (Ran-GTP in nucleus, Ran-GDP in cytosol)

What does Ran-GAP do?

In cytosol; converts Ran-GTP → Ran-GDP

What does Ran-GEF do?

In nucleus; converts Ran-GDP → Ran-GTP

What is the nuclear lamina?

Mesh of lamins providing shape/stability to nuclear envelope

How does the nuclear envelope disassemble during mitosis?

Phosphorylation of lamins/nucleoporins; dynein tears envelope off chromatin

How is the nuclear envelope reassembled?

Ran-GTP near chromatin releases NPC proteins → reassembly in daughter nuclei

How are mitochondrial and chloroplast proteins made?

Synthesized in cytosol and imported post-translationally

What are TOM/TIM complexes?

Translocators for protein import across mitochondrial membranes

What is the SAM complex?

Folds beta-barrel proteins into mitochondrial outer membrane

What drives mitochondrial protein import?

ATP hydrolysis + membrane potential

What does mitochondrial hsp70 do?

Chaperone that pulls proteins into matrix using ATP

What are peroxisomes?

Single membrane organelles with oxidative enzymes; import proteins via Pex5

How are peroxisomes formed?

Budding from ER → fusion → import of peroxisomal proteins

What is the endoplasmic reticulum (ER)?

Network of tubules/sacs continuous with nuclear membrane; site of lipid & protein synthesis

What is rough ER?

Studded with ribosomes; site of co-translational protein import

What is smooth ER?

Lacks ribosomes; makes lipids; detoxifies; stores Ca2+

What are microsomes?

Vesicles from fragmented ER used to study ER functions

What does the signal-recognition particle (SRP) do?

Binds ER signal sequence and pauses translation until ribosome binds ER membrane

What is the SRP receptor?

Membrane receptor that docks SRP–ribosome complex to ER

What is the Sec61 complex?

Core ER translocator forming gated channel for polypeptide passage

What is a translocon?

Complex of translocator + accessory enzymes (e.g.

What are single-pass membrane proteins?

Inserted by start-transfer and stop-transfer sequences; anchored in membrane

What are multi-pass membrane proteins?

Alternate start- and stop-transfer sequences create multiple transmembrane regions

What are ER tail-anchored proteins?

C-terminal helix anchors them; inserted post-translation using Get3 ATPase system

What is Protein Disulfide Isomerase (PDI)?

ER enzyme forming disulfide bonds in lumenal proteins

What is BiP?

ER chaperone that pulls proteins into ER and retains misfolded ones

What is glycosylation?

Attachment of oligosaccharides to proteins

What is N-linked glycosylation?

Transfer of oligosaccharide to asparagine side chain; catalyzed by oligosaccharyl transferase

What is dolichol?

Lipid anchor for oligosaccharides in ER membrane

What is O-linked glycosylation?

Attachment to serine/threonine hydroxyl groups; continues in Golgi

What do calnexin and calreticulin do?

ER chaperones binding Ca2+ and assisting glycoprotein foldingWhat drives nuclear transport in the appropriate direction?

What are the two forms of Ran?

Ran-GTP (nucleus) and Ran-GDP (cytosol)

Where is Ran-GEF located?

In the nucleus bound to chromatin

Where is Ran-GAP located?

In the cytosol

What creates the directionality of nuclear transport?

The Ran-GTP/Ran-GDP gradient

What causes release of cargo in nuclear import?

Ran-GTP binding to importin in the nucleus

What causes cargo release in nuclear export?

Hydrolysis of Ran-GTP to Ran-GDP in the cytosol

What do importins recognize?

Nuclear localization signals (NLS)

What structures mediate nuclear transport?

Nuclear pore complexes (NPCs)

What are NPCs made of?

Nucleoporins

What do FG repeats in NPCs do?

Facilitate transient interactions with transport receptors

What do exportins recognize?

Nuclear export signals (NES)

What molecule binds exportin and cargo together in the nucleus?

Ran-GTP

How are most mitochondrial proteins encoded?

By nuclear DNA and synthesized in the cytosol

What directs proteins to mitochondria?

N-terminal signal sequences

What complex recognizes mitochondrial import signals on the outer membrane?

TOM complex

What complexes transport proteins across or into the inner mitochondrial membrane?

TIM complexes

What energy sources drive mitochondrial protein import?

ATP and membrane potential (Δψ)

What happens to the signal sequence after mitochondrial import?

It is cleaved off

What helps proteins fold inside mitochondria?

Matrix chaperones (Hsp70)

Where are peroxisomal proteins synthesized?

In the cytosol on free ribosomes

What do peroxisomal targeting signals (PTS) do?

Direct proteins to peroxisomes

What recognizes peroxisomal targeting signals?

Pex proteins (peroxin receptors)

Do peroxisomes import folded or unfolded proteins?

Fully folded proteins

What energy is required for peroxisomal import?

ATP

How can peroxisomes form?

By growth/division of existing peroxisomes or de novo from ER vesicles

Where are proteins for secretion and membrane insertion synthesized?

On ribosomes bound to the ER

What directs ribosomes to the ER membrane?

Signal sequences and SRP (signal recognition particle)

What does SRP do?

Binds signal sequence

What receptor binds SRP?

SRP receptor on the ER membrane

What complex allows proteins to enter the ER?

Sec61 translocator

What enzyme removes the ER signal sequence?

Signal peptidase

Where do soluble proteins end up after ER translocation?

In the ER lumen

Where do membrane proteins remain after ER translocation?

Embedded in the ER membrane

What stops translocation of single-pass membrane proteins?

A stop-transfer sequence

What determines the orientation of transmembrane proteins?

Charge distribution around signal sequence

What determines the number of membrane spans in a protein?

Multiple signal and stop-transfer sequences

What mediates transport between compartments?

Vesicles

What maintains distinct organelle composition?

Selective vesicular transport

What proteins shape vesicles?

Coat proteins