01.25.10 Lecture 6 - Intracellular compartments and transport I
Intracellular transport and compartments 1. Protein sorting: How proteins get to their appropriate destinations within the cell 2. Vesicular transport: How vesicles shuttle proteins and membranes between cellular compartments
Primary functions of the compartments within a cell
Relative cellular volumes of the major membranous organelles
The evolution of organelles: nuclear membranes and ER
The evolution of organelles: mitochondria
Organelles import proteins by three distinct mechanisms 1. Transport from the cytoplasm into the nucleus through nuclear pores 2. Transport from the cytoplasm to organelles by protein translocators in the membrane 3. Transport from ER to other organelles occurs via vesicles
Signal sequences target proteins to their destinations
Signal sequences are necessary and sufficient for protein targeting
Mechanism 1: proteins enter the nucleus via nuclear pores The nuclear envelope is a double membrane Contiguous with the ER - both compartments share the same lumen Perforated by nuclear pores
The nuclear pore complex (NPC) is a selective molecular gate Composed of ~100 different proteins Small, water-soluble molecules pass freely, macromolecules must carry appropriate signal
NPCs actively transport proteins bound for the nucleus 1. Proteins bind to nuclear transport receptors 2. Complex is guided to the pore by filaments 3. Pore opens, receptor + protein are transported in (uses GTP) 4. Receptor is shuttled back into the cytoplasm
The nuclear envelope disassembles and reforms during each cell division
Mechanism 2: protein translocation from cytoplasm to organelle Proteins moving from the cytosol into the ER, mitochondria, chloroplasts, or peroxisomes Protein movement is mediated by specialized proteins termed protein translocators Unlike passage through nuclear pores, translocation requires unfolding or cotranslational transport
Proteins are unfolded during translocation into mitochondria
The ER is a network of membrane sheets & tubules
Active ribosomes may be in the cytosol or associated with the ER
Ribosomes are directed to the ER by the SRP and ER signal
Soluble proteins cross the ER membrane into the lumen
Soluble proteins cross the ER membrane into the lumen
Integration of transmembrane proteins into the ER membrane
Multi-pass proteins use internal starttransfer sequences
Mechanism 3: vesicular transport
Transport vesicles Continually bud off from and fuse to other membrane compartments producing a constant flux of material Carry soluble proteins (in the lumen) and lipids & membrane proteins (in the bilayer) between compartments Are transported along microtubules by motor proteins
Vesicle budding is driven by assembly of a protein coat
Clathrin-coated vesicles transport selected cargo molecules
Complexes of clathrin form a basket around vesicles and help them to pinch from membranes
Step 1 Cargo molecules (red) bind to transmembrane cargo receptors Cytoplasmic domains of receptors bind to adaptin (light green)which recruits clathrin Clathrin clusters cargo/ receptor/adaptin complexes and induces curvature to the membrane - clathrin-coated pit
Step 2 Additional clathrin molecules bind - increasing curvature Dynamin assembles a ring around each clathrincoated pit
Step 3 Dynamin rings constrict to pinch the membrane off Dynamin is a GTPase and used the energy released from GTP hydrolysis to power this reaction
Step 4 The free vesicle sheds its coat of adaptin and clathrin Vesicles are transported to their destination on microtubules
Clathrin-coated vesicles transport selected cargo molecules
Clathrin-coated vesicles transport selected cargo molecules
Animation of clathrin assembly and disassembly around an endocytic vessicle
SNAREs are proteins that target vesicles to specific compartments v-snares are on vesicles t-snares are on target compartments
SNARE proteins are important for membrane fusion v-snares and t-snares bind tightly Complexes bring the two membranes together to promote fusion