This presentation aims to provide an overview of the mechanisms and key players involved in intracellular transport, highlighting their significance in cellular processes. Overview of Cellular Organization: Briefly describe the different compartments of a eukaryotic cell, including the nucleus, cytoplasm, endoplasmic reticulum, Golgi apparatus, mitochondria, and lysosomes. Emphasize that intracellular transport ensures the efficient distribution of components between these compartments.
3. 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
4. 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
Essential cell biology by Bruce Alberts, et al.
6. Signal sequences target proteins to
their destinations
Essential cell biology by Bruce Alberts, et al.
7. Signal sequences are necessary and
sufficient for protein targeting
Essential cell biology by Bruce Alberts, et al.
8. 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
Essential cell biology by Bruce Alberts, et al.
9. 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
Essential cell biology by Bruce Alberts, et al.
10. 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
NPCs actively transport proteins
bound for the nucleus
Essential cell biology by
Bruce Alberts, et al.
11. Energy supplied by GTP Hydrolysis
drives nuclear transport
Essential cell biology by Bruce Alberts, et al.
12. 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 co-translational transport
18. Multi-pass proteins use internal start-
transfer sequences
Essential cell biology by Bruce Alberts, et al.
19. Some proteins are retained in the ER
• Proteins containing ER retention signal are held in the ER lumen (i.e. for disulfide
bond formation, oligosaccharide transferases)
• Proteins that are misfolded or fail to oligomerize are held for quality control by
chaperonins
Essential cell biology by Bruce Alberts, et al.
20. Most (or all) proteins are covalently
modified in the ER
1. Formation of disulfide bonds between cysteine residues of
the same polypeptide (intra-molecular) or different
polypeptides (inter-molecular)
2. Addition of short oligosaccharide side chains (glycosylation)
21. Many proteins are glycosylated in the
ER
1. An appropriate asparagine
enter the ER lumen.
2. Branched oligosaccharide
(14 sugars) transferred
from dolichol (a specialized
lipid) by enzyme
oligosaccharyl transferase.
3. Sequence contains
asparagine – X – Serine or
asparagine – X – threonine
Essential cell biology by Bruce Alberts, et al.
22. Proteins are unfolded during
translocation into mitochondria
Essential cell biology by Bruce Alberts, et al.
25. 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
Essential cell biology by Bruce Alberts, et al.
26. Vesicle budding is driven by
assembly of a protein coat
Essential cell biology by Bruce Alberts, et al.
27. Complexes of clathrin form a basket around
vesicles and help them to pinch from
membranes
Essential cell biology by Bruce Alberts, et al.
28. Clathrin-coated vesicles transport
selected cargo molecules
• 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
• Additional clathrin
molecules bind -
increasing curvature
• Dynamin assembles a
ring around each
clathrin-coated pit
• Dynamin rings
constrict to “pinch”
the membrane off
• Dynamin is a GTPase
and used the energy
released from GTP
hydrolysis to power
this reaction
• The free vesicle sheds its
coat of adaptin and
clathrin
• Vesicles are transported
to their destination on
microtubules
Essential cell biology by Bruce Alberts, et al.
29. SNAREs are proteins that target
vesicles to specific compartments
v-SNAREs are on
vesicles
t-SNARES are on
target compartments
Essential cell biology by Bruce Alberts, et al.
30. SNARE proteins are important for
membrane fusion
• v-SNAREs and t-SNAREs bind tightly
• Complexes bring the two membranes together to promote
fusion
Essential cell biology by Bruce Alberts, et al.
32. 1.The secretory pathway: how newly made lipids, proteins, and carbohydrates
are delivered to the cell surface via exocytosis
2.Endocytic pathways: how cells take up fluids and particles (large and small)
from the extracellular environment
Exocytosis & Endocytosis
33. Proteins are further modified in the
Golgi apparatus
• Proteins enter the Golgi via vesicle fusion with the cis face and pass through the
Golgi between successive stacks by transport vesicles
• Oligosaccharide chains added in the ER are modified by enzymes in the Golgi -
addition and removal of sugars to make complex oligosaccharides
• Reactions are processive - early-acting enzymes in the cis compartments, late
acting enzymes in the trans compartments
34. The Golgi apparatus is made of
stacked, flattened membranes
Essential cell biology by Bruce Alberts, et al.
35. The Roles of Golgi
apparatus
Essential cell biology by Bruce Alberts, et al.
36. 2 pathways for exocytosis: regulated
and constitutive
Essential cell biology by Bruce Alberts, et al.
37. Exocytosis occurs via 2 distinct
pathways: regulated and constitutive
• Constitutive exocytosis: A steady stream of delivery occurring in all cells. Plasma
membrane components to replace endocytosed material and for membrane
growth.
• Regulated exocytosis: Operates only in cells specialized for secretion (i.e. secretory
cells in the gut and glands). Secretory vesicles are docked at the plasma
membrane until the cells receive an outside signal.
38. The endocytic pathway:
Two main mechanisms of endocytosis:
• Phagocytosis: “cellular eating”; The ingestion of large particles (i.e. microorganisms,
cellular debris) via large vesicles called phagosomes. Only occurs in specialized cells
• Pinocytosis: “cellular drinking”; The ingestion of fluid and small molecules via small
(<150 nm diameter) vesicles. Occurs in all cells.
40. Receptor-mediated endocytosis is a
specialized form of pinocytosis
• Pinocytosis traps molecules in the extracellular fluid randomly
• Receptor-mediated endocytosis traps specific molecules, concentrates them in
vesicles
• Both processes use clathrin-mediated vesicle formation
