Protein Sorting and Principles of Vesicular Transport

L 13 Molecular & Cell Biology

Translation starts on free ribosomes

All proteins begin synthesis in cytosol

Post-translational import

Protein synthesis finishes in cytosol before import

Co-translational import

Protein finishes synthesis at ER

ER signal sequence

Hydrophobic N-terminal stretch

Signal sequence purpose

Acts as ZIP code directing protein destination

Gated transport

Movement through nuclear pore complex

Nuclear pore complex structure

Cytosolic fibrils + central channel + nuclear basket

Nuclear localization signal (NLS)

Lys/Arg-rich basic sequence

Importin

Receptor that binds NLS

Exportin

Receptor that binds NES

Ran-GTP location

High in nucleus

Ran-GDP location

High in cytosol

Ran-GEF

Converts Ran-GDP → Ran-GTP in nucleus

Ran-GAP

Converts Ran-GTP → Ran-GDP in cytosol

Cargo release (import)

Ran-GTP binds importin in nucleus

Cargo release (export)

Ran-GTP hydrolysis releases cargo in cytosol

NLS regulation

Hidden by phosphorylation or inhibitor protein

Energy for nuclear transport

GTP (Ran cycle)

Mitochondrial import state

Proteins imported unfolded

Cytosolic Hsp70

Keeps mitochondrial precursors unfolded

Presequence

Mitochondrial targeting signal

TOM complex

Outer membrane translocase

TIM23 complex

Inner membrane translocase for matrix proteins

Matrix processing peptidase (MPP)

Cleaves presequence in matrix

Energy for mitochondrial import

ATP + proton-motive force (Δψ)

Mitochondrial Hsp70

Pulls proteins into matrix using ATP

Mitochondrial DNA protein number

13 proteins encoded

SAM complex

Inserts β-barrel proteins into OMM

TIM22 complex

Inserts multipass inner membrane proteins

Transit peptide

Chloroplast targeting signal

TOC complex

Outer membrane translocase

TIC complex

Inner membrane translocase

Chloroplast import state

Proteins imported unfolded

SPP

Cleaves transit peptide in stroma

Δψ in chloroplasts

Only across thylakoid membrane

Energy for chloroplast import

ATP/GTP (no Δψ in inner envelope)

Tat pathway

Moves fully folded proteins using ΔpH

Sec pathway

Moves unfolded proteins

SRP pathway

Co-translational insertion

Spontaneous insertion

Direct membrane insertion with no machinery

Peroxisome import state

Proteins imported folded

PTS1 signal

SKL C-terminal tripeptide

Pex5

Receptor for PTS1 proteins

PTS1 cleavage

Signal not removed after import

Energy for peroxisome import

ATP

Mitochondrial membranes

Two membranes

Chloroplast membranes

Three membranes (outer, inner, thylakoid)

Δψ location in mitochondria

Inner mitochondrial membrane

Δψ location in chloroplasts

Thylakoid membrane

Mito import machinery

TOM/TIM

Chloro import machinery

TOC/TIC

Mito signal peptide

Presequence

Chloro signal peptide

Transit peptide

Protein folding location mitochondria

Matrix

Protein folding location chloroplasts

Stroma