Intracellular Transport

Three basic modes of transport 1. Gated transport 2. Transmembrane transport

3. Vesicular transport

Alberts

Vesicular transport

A Simplified Roadmap of Intracellular Transport

Overview of major protein sorting pathways in eukaryotic cells

Protein sorting

mitochondria

rough

ER

peroxisomes

cytosol

nucleus

plasma

membrane Golgi

lysosomes

nuclear

envelope

smooth

ER

secreted

Mechanism 1: Gated Transport,

proteins enter the nucleus via nuclear pores

The nuclear envelope is a double membrane

Continuous with the ER - both compartments share the same lumen

Perforated by nuclear pores

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 Mechanism of Nuclear Transport

Alberts

Vesicular transport


Mechanism 2: Transmembrane Transport,

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

Proteins are unfolded during translocation

into mitochondria

Alberts

Vesicular transport


Mechanism 3: Vesicular Transport

Vesicular transport delivers components between compartments in the biosynthetic-secretory and endocytic pathways.

Vesicular transport delivers components between compartments in the biosynthetic-secretory and endocytic pathways.

Vesicular transport

Two Key Steps: 1. Sorting during vesicle formation 2. Targeting during vesicle fusion

14

endocyto

sis

exocyto

sis

(secre

tion)

Retr

ogra

de t

raffic

Plasma membrane

lysosome

sorting endosome

recycling endosome

late endosome

trans-Golgi

medial-Golgi

cis-Golgi

endoplasmic reticulum

EGTC

secretory vesicles

TGN

Nucleus

Cellular Membranes Endomembrane Structure % total membrane in

Hepatocyte

% total in Pancreatic

Exocrine Cell

Endoplasmic Reticulum

(smooth)

16

What is the endomembrane system?

System of membrane-bound organelles in cells that work cooperatively together to create secretory proteins, membrane-bound proteins, or plasma membrane proteins

The endomembrane system regulates protein traffic and performs metabolic functions in the cell

The set of membranes that form a single functional and developmental unit, either being connected directly, or exchanging material through vesicle transport.

Also involved in assembly and transportation of lipids

Properties of endomembrane system

Present only in eukaryotic cells

Transport system for moving molecules through interior of the cell

Made of lipid bi-layer with proteins attached to either side or transversing them

Divides cell into organelles

The endomembrane system plays a key role in the synthesis (and hydrolysis) of macromolecules in the

cell.

The various components

modify

macromolecules

for their various

functions.

Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 7.16

Endomembrane System

secretory vesicles

Endoplasmic reticulum(ER)

Golgi complex

Lysosome

The endomembrane system consists of:

The Endomembrane System

1. The endoplasmic reticulum manufactures membranes and performs many other

biosynthetic functions

2. The Golgi apparatus finishes, sorts, and ships cell products

3. Lysosomes are digestive compartments

4. Vesicles encloses and transport substances synthesized in the cell.

Vacuoles are membrane-bound sacs with varied functions.

Food vacuoles, from phagocytosis, fuse with lysosomes.

Contractile vacuoles, found in freshwater protists, pump excess water out of the cell.

Central vacuoles are found in many mature plant cells.

Vacuoles have diverse functions in cell

maintenance

Vacuole filling with water

Vacuole contracting

(a) Contractile vacuole in Paramecium

(b) Central vacuole in a plant cell

Central vacuole

Co

lori

zed

TEM

LM

LM

Figure 4.17 Laura Coronado Bio 10 Chapter 4

Endoplasmic Reticulum ENDOPLASMIC RETICULUM (ER):

An extensive

tubovesicular network

where proteins and

lipids are made.

Rough ER: studded with

ribosomes, site of protein

biosynthesis

Smooth ER: site of lipid

biosynthesis

Rough ER Smooth ER

Endoplasmic Reticulum

There are two distinct regions of ER Smooth ER, which lacks ribosomes Rough ER, which contains ribosomes

Functions of ER

The smooth ER

Synthesizes lipids

Metabolizes carbohydrates

Stores calcium (In muscle cells, these trigger contractions)

Detoxifies poison

The rough ER

Has bound ribosomes

Produces proteins and membranes, which are distributed by transport vesicles

Smooth Rough

The synthesis of phosphatidylcholine in the ER membrane

Rough ER Functions

Protein and Membrane synthesis

Ribosomes covering Rough ER secrete proteins

Folded into lumen Later transported by vesicles

Ex. Insulin

Membranes made for itself are later transported to other endomembrane systems

Synthesis of secretory proteins - review

1. N-terminal signal sequence is synthesized

2. Signal bound by SRP, complex docks with SRP receptor on ER membrane

3. Signal sequence binds to translocon, internal channel opens, inserted into translocon

4. Polypeptide elongates, signal sequence cleaved

5. ER chaperones prevent faulty folding, carbohydrates added to specific residues

6. Ribosomes released, recycle

7. C-terminus of protein drawn into ER lumen, translocon gate shuts, protein assumes final conformation

Synthesis of secretory proteins

Synthesis of integral membrane protein

1. internal signal sequence bound by SRP

2. SRP-protein-ribosome

complex docks with SRP

receptor, C-terminal portion

of protein cotranslationally

inserted into lumen of ER

3. Mature protein transverses ER bilayer forming integral membrane protein

NOTE: Orientation of protein within membrane dependent upon cluster of charged residues adjacent to internal signal sequence

4. Polypeptide elongates, carbohydrates added to specific residues

5. Ribosomes released, recycle, integral membrane protein produced that forms transmembrane domain

Synthesis of membrane proteins

PROTEIN GLYCOSYLATION IN THE ROUGH ER: During

translation, a signal sequence on membrane and secretory

proteins directs the nascent protein into the ER lumen. After the

protein has entered the ER, the glycosylation process begins.

A PRE-FORMED

PRECURSOR

OLIGOSACCHARIDE

IS TRANSFERRED

EN BLOC

TO PROTEINS

IN THE ER

Alberts

PROTEIN GLYCOSYLATION IN THE ROUGH ER

Alberts

SOME PERIPHERAL MEMBRANE PROTEINS

AQUIRE A COVALENTLY ATTACHED

GLYCOPHOSPHATIDYLINOSITOL (GPI)

ANCHOR IN THE ER

Alberts

Transport from the ER through the Golgi apparatus

Golgi Apparatus

Made of cisternae Cis and Trans faces

Cis serves as bridge w/ER Trans makes vesicles for transport to other cell regions

Modifies proteins Cisternae between cis and trans faces

Works in partnership with the ER Receives, refines, stores, and distributes chemical products of the cell

12/28/2014 41

Function

1. First modification of lipids and proteins (Modifies the N-linked oligosaccharides and adds O-linked oligosaccharides)

2. Storage and packaging of materials that will be exported from the cell.

OLIGOSACCHARIDE CHAINS ARE PROCESSED

IN THE GOLGI APPARATUS

Production of complex oligosaccharides

Modification of the N-linked oligosaccharides is done by enzymes in the lumen of various Golgi compartments.

Two possible models

explaining the organization

of the Golgi apparatus and

the transport of proteins from

one cisterna to the next

Transport from the trans Golgi nextwork to Lysosomes

The Lysosomes

The structure of the lysosome:

Discovered in 1950 by Rene . De .Duve, a Lysosome is a tiny

membrane-bound organelle found in the cytoplasm of all

eukaryotic cells containing various acid hydrolytic enzymes

that can digest every kind of biological molecule.

The structure of the lysosome

Lysosomes is common in

animal cells but rare in plant.

Marker enzyme: acid

phosphatase.

Lysosomes are highly heterogeneous Shape and size But all have acid hydrolases The stomach of a cell

The structure of the lysosome

Lysosome membrane: 1.H+-pumps:

internal proton is kept high H+-

concentration by H+-ATPase

2.Glycosylated proteins:

may protect the lysosome from

self-digestion.

3.Transport proteins:

transporting digested materials.

Biogenesis of Lysosomes

1. A phosphate attached to the mannose residue.

2.This mannose-6 phosphate forms a sorting signal that moves through the

cisternae to the trans region where it binds to a specific receptor.

3.After it binds to the receptor, it begins to bud and a coat made of clathrin

forms around the bud (to strengthen it).

4.It moves away to fuse with a late endosome .

5.The phosphate is removed and hydrolase is dissociated from the receptor.

6.The receptor is then recycled back to the Golgi complex .

Biogenesis of Lysosomes

RER Cis golgi

network Trans golgi

network

Golgi apparatus

Lysosomal hydrolase precursor

Addition of phosphate

Mannose-6-phosphate(M-6-P)

Mature

lysosomal

hydrolase

ATP ADP+Pi

H+

PH=5

Binding to

M6P receptor

From

RER

Mature lysosomes

Dissociation at

acidic pH

Removal of

phosphate

Late endosome

M6P receptor in

budding vesicle

Receptor-dependent

transport

Clathrin coat

The types of lysosomes

Primary lysosome are newly formed by

budding from the Golgi complex,and

therefore have not yet encountered

substrate for digestion and with acid

Hydrolytic enzymes inactive.

Primary Lys.

Second Lys

Secondary lysosomes result from the

repeated fusion of primary lysosomes

with a variety of membrane bounded

substrates and active hydrolytic enzymes

within the lysosomes. The bounded

substrates may be food bacteriumor worn organalles and so on.

Phagosome is a kind of secondary

lysosomes licked up food or bacterium.

Autophagosome is a kind of secondary

lysosomes licked up ageing organelles.

The secondary lysosomes digest the contents of phagocytic or autophagic vesicles to

form residual bodies that either undergo exocytosis or are retained in the cell as

lipofuscin granules.

D. The Functions of Lysosomes

The functions of Lysosomes

Lysosomes are involved in four major cell functions:

1.Heterophagy;

2.Autophagy;

3.The extracellular digest;

4. Autocytolysis;

All major classes of macromolecules are degraded in lysosomes

The functions of Lysosomes 1. Heterophagy

Digestion of materical of extracellular origin.

Lysosomes pick up foreign invaders such as bacteria, food and break them into small pieces that can hopefully be used again. If they pick up a really harmful invader, they will eat it up and expel what is left of it out of the cell so that the debris can be removed from the body.

2.Autophagy Digestion of materical of intracellular

origin. Lysosomes also play a key role in destroying old organelles within the cell and thus allow them to be replaced with fresher, more effective ones.This process is known as autophagy and is accomplished in two stages.


Autophagy Firstly, a membrane is

donated by the endoplasmic reticulum. This membrane then surrounds the old organelle.

Secondly ,a lysosome fuses with this membrane to form an autophagic vacuole. The lysosome can safely enter it's enzyme contents into this vacuole and destroy the old organelle. The electron micrograph shows a lysosome in the process of destroying a membrane bound mitochondria.

3.The extracellular

digest :Another function of

lysosome in the human

occurs during fertilization

of the egg by the sperm.

The head of the sperm cell

contains a package of

lysosomal material called

the acrosome.


The enzymes from this are

released when the sperm

makes contact with an egg

and they effectively bore a

hole through the cell

membrane of the egg

allowing the sperm to enter.


4.Autocytolysis :Lysosomes may also be important in

development. For instance , they are responsible for the

breakdown of a tadpoles tail as the tadpole develops into a

frog. In the process, the lysosome releases hydrolases to

cytoplasm to digest the cell of oneself.

Transport into the cell from the plasma membrane endocytosis

Endocytosis: process of taking in liquids or larger molecules into a cell by engulfing in a vesicle;

requires energy

Endocytosis

Phagocytosis by a macrophage Phagocytosis by a neutrophil

Pinocytosis: cell drinking; Phagocytosis: cell eating

pseudopods

One macrophage and two red blood cells

Phagocytosis: large particle, >250nm

Pinocytosis: fluid, liquid, 100 nm

ENDOCYTOSIS IS IMPORTANT FOR

CELLS TO:

1. Import selected extracelluar molecules (i.e.

receptor-mediated endocytosis).

2. Regulate levels of membrane proteins on the

cell surface (i.e. receptor down-regulation).

Transport from the trans Golgi network to the cell exterior: exocytosis

Exocytosis: fusion of vesicles with the plasma membrane

Exocytosis

Exocytosis: process of releasing substances out of a cell by fusion of a vesicle with the membrane

Exocytosis of

secretory vesicles

Two mechanisms of secretion

Three pathways of protein sorting in the trans Golgi network

Documents

Intracellular Transport