Lec14 CH15 Intracellular Transport I

Gated transport

Folded proteins enter through pores.

Transmembrane transport

Proteins (often unfolded) enter via protein translocators.

Vesicular transport

Folded proteins move via transport vesicles that fuse with destination membranes.

Signal sequence

Continuous stretch of 3 - 60 amino acids necessary and sufficient to direct proteins to the correct destination.

Nucleus

Contains nuclear genome, DNA & RNA synthesis.

Endosome

Sorting of endocytosed material.

Mitochondria

ATP synthesis by oxidative phosphorylation.

Cytosol

Metabolic pathways, protein synthesis.

Lysosome

Intracellular degradation.

Golgi apparatus

Protein and lipid modification and sorting.

Endoplasmic reticulum

Rough: protein synthesis for distribution; Smooth: lipid synthesis.

Peroxisome

Oxidative breakdown of toxic molecules.

Free ribosomes

For protein synthesis in the cytosol.

Chloroplast

ATP synthesis and carbon fixation by photosynthesis.

Protein sorting

The process of directing proteins to their correct destination within the cell.

Ran-GTP

A protein involved in nuclear import, facilitating the transport of proteins into the nucleus.

Ran-GDP

The inactive form of Ran, which is converted to Ran-GTP to assist in nuclear transport.

Ran GEF

A guanine nucleotide exchange factor that promotes the conversion of Ran-GDP to Ran-GTP.

Ran GAP

A GTPase-activating protein that facilitates the hydrolysis of Ran-GTP to Ran-GDP.

Nuclear pore complex (NPC)

A large protein complex that spans the nuclear envelope and regulates the transport of molecules between the nucleus and cytoplasm.

Signal-recognition particle (SRP)

A ribonucleoprotein that recognizes and targets specific proteins to the endoplasmic reticulum.

Translocators

Protein complexes that facilitate the import of proteins across membranes.

Energy in import

Required for the processes of protein transport through translocators.

Nuclear pores

Conduct extensive bidirectional traffic between the cytosol and the nucleus.

Outer nuclear membrane

Continuous with the ER membrane.

Nuclear pore complex

Forms a gate through which selected macromolecules and larger complexes enter or exit the nucleus.

Nuclear localization signal

Folded proteins with this signal are recognized by nuclear import receptors.

Nuclear import receptors

Recognize folded proteins with nuclear localization signal.

Cytosolic fibrils

Protein+receptor interact with these during nuclear transport.

Gel-like meshwork

Protein+receptor pass through this structure of nuclear pore proteins.

GTP hydrolysis

Energy supplied by this process drives nuclear transport.

GTPase Ran

Binds to the nuclear transport receptor and returns it to the cytoplasm.

Ran-GAP

GTPase activating protein that mediates the activity of Ran.

Ran-GEF

Guanine exchange factor that mediates the activity of Ran.

mRNA export

Some proteins travel with the mRNA as it moves through the pore, while others remain in the nucleus.

Energy source for mRNA export

Primarily provided by the hydrolysis of GTP and ATP.

Transmembrane import

Mitochondrial (and chloroplast) precursor proteins are unfolded during this process.

Chaperones

Facilitate the targeting and import of proteins into the mitochondria.

Mitochondrial signal sequence

Recognized by a receptor in the outer mitochondrial membrane.

Protein translocator (TOM)

Transports the mitochondrial signal sequence to the intermembrane space.

Second translocator (TIM)

Recognizes the signal sequence and transports the protein across the inner membrane.

Signal peptidase

Cleaves the sequence signal in the matrix.

Peroxisomes

Small, membrane-bound organelles found in nearly all eukaryotic cells.

Peroxin receptor proteins (Pex5)

A short sequence of 3 amino acids is recognized by these proteins in the cytosol, which bring cargo proteins to the peroxisome.

Peroxin translocator complex (Pex14)

Aids in transmembrane transport of the protein into the peroxisome; the protein remains folded.

ATP hydrolysis

Energy supplied by ATP hydrolysis drives peroxisomal transport.

Co-translational import

ER proteins are synthesized as they are transported.

Signal recognition particle (SRP)

An ER signal sequence and this particle direct a ribosome to the ER membrane.

SRP receptor

The SRP binds to this receptor at the ER membrane, and is then passed to a protein translocator.

Single-pass transmembrane protein

A protein retained in the lipid bilayer determined by start and stop signals.

Double-pass transmembrane protein

A protein that has an internal ER signal sequence.

Lipid-transfer proteins

Lipids are synthesized in smooth ER and transferred to other organelles mostly via transport vesicles.

Transport vesicles

Used for transferring lipids from smooth ER to other organelles (except mitochondria).

Membrane contact sites

Lipid transfer between the ER and mitochondria involves these sites and lipid-transfer proteins.

iClicker Question

How many times does this protein cross the ER membrane? A) 1 B) 2 C) 3 D) 4 E) 5

Transporters for mitochondria

What transporters are represented by __[1]__ and __[2__]? A. [1] = TOM, [2] = TIM B. [1] = TIM, [2] = TAM C. [1] = TIM, [2] = TOM D. [1] = TAM, [2] = TIM E. None of the above