Intracellular Compartments and Protein Sorting

Compartmentalization of Cells

Pages 695-712

Proteins Characterize Organelles

-They catalyze the reactions that occur in each organelle

-They selectively transport small molecules in and out of its interior

-They serve as organelle-specific surface markers that direct new deliveries of proteins and lipids to the appropriate organelle

Major Intracellular Compartments

Organelle Function

Nucleus – Contains the genome and is the site for DNA and RNA synthesisEndoplasmic Reticulum – Produces most of the lipid for the rest of the cell

-Functions in transport of proteins to the Golgi-Functions as a store for calcium

Golgi Apparatus – Receives proteins and lipids from the ER and dispatches them to several destinationsMitochondria – Generates most of the ATP used by cellsLysosomes – Degrades intracellular organelles and

macromolecules taken in from outside the cellEndosomes – Contain material taken in from outside the cellPeroxisomes – Contain enzymes involved in oxidative reactions

TABLE 12–1 Relative Volumes Occupied by the Major IntracellularCompartments in a Liver Cell (Hepatocyte)

INTRACELLULAR COMPARTMENT PERCENTAGE OF TOTAL CELL VOLUME

Cytosol 54Mitochondria 22Rough ER cisternae 9Smooth ER cisternae plus Golgi cisternae 6Nucleus 6Peroxisomes 1Lysosomes 1Endosomes 1

EM of a Liver Cell

-Organelles often have characteristic positions in the cytosol depending on interactions with the cytoskeleton.

-The ER and Golgi depend on the microtubule array.

-Eucaryotic cells are 10-20 times larger linearly, but 1,000-10,000 times greater in volume.

Specialization of Membrane Function

Development of Plastids

Evolution of Cell Nucleus and ER

Evolution of Mitochondria

Relationships between Compartments

Protein Traffic Map

Types of Protein Transport

1. Gated transport – Protein traffic between the nucleus and cytosol occurs between topographically equivalent spaces, which are connected through the nuclear pore complexes

2. Transmembrane transport – Membrane-bound protein translocators directly transport specific proteins across a membrane from the cytosol into a space that is topologically distinct

3. Vesicular transport – Membrane-enclosed transport intermediates ferry proteins from one compartment to another

Vesicle Transport

Types of Sorting SignalsEach type of protein transfer is usually guided by sorting signals in the transported protein that are recognized by receptors. Most receptors recognize classes of proteins rather than just one protein.

15-60 AA

Signal Sequences

Organelles Cannot be Constructed from Scratch

During division, cells must duplicate their organelles

They do it by enlarging existing organelles by incorporating new molecules into them and then dividing

Each daughter cell inherits their organelles from their mother

Nuclear Envelope

Defines the nuclear compartment

Inner membrane contains specific proteins that interact with chromatin and the nuclear lamina

Outer membrane is continuous with the membrane on the ER

Nuclear Pore Complexes

Nuclear side

-Composed of over 30 different proteins called nucleoporins-The more active the nucleus is in transcription the more complexes the envelope contains, typically there’s 3000-4000

Nuclear Pore Complex

500 macromolecules per second

Free Diffusion through Nuclear Pores

9 nm diameter limit for free diffusion, but up to 39 nm can be brought through by transport receptors

Function of a NLS

NLS is rich in positively charged amino acids, lysine and arginine

Nuclear proteins can be transported through a pore complex while they are in a fully folded conformation

Visualizing Active Transport

Nuclear Import Receptors

The import receptors are soluble cytosolic proteins that bind both the NLS on the protein to be transported and to nucleoporins. Many of the nucleoporins contain phenylalanine-glycine (FG)-repeats that serve as binding sites for the import receptors

Nuclear Export

Relies on nuclear export signals on proteins that are bound by nuclear export receptors

Both types of receptors belong to the family of nuclear transport receptors

In yeast there are 14 genes in this family, many more in humans

A single pore complex conducts traffic in both directions

Import into nucleus -Pro-Pro-Lys-Lys-Lys-Arg-Lys-Val-Export from nucleus -Leu-Ala-Leu-Lys-Leu-Ala-Gly-Leu-Asp-Ile-

Compartmentalization of Ran-GDP and Ran-GTP

GAP – GTPase-activating protein

GEF – Guanine exchange factor

Ran is a GTPase

Directionality of Nuclear Transport

Ran BindingProtein

Model for Cargo Release by Ran-GTP

Nuclear Transport in Drosophila

-Gene regulatory protein called dorsal stained brown

-Expressed in the ventral nuclei

Control of Nuclear Import during T-cell Activation

NF-AT – Nuclear factor of activated T cells

Regulation of nuclear localization is done by phosphorylation

The Nuclear Lamina

Nuclear lamins - are Intermediate filaments

-Gives shape and stability to the nuclear envelope

-Interacts with chromatin

Assemble/Disassembly of the Nuclear Lamina

-Lamin Phosphorylation

-NPCs disassemble, disperse, bind nuclear import receptors

-Motor proteins are involved with disassembly

-Nuclear envelope reassembles around chromosomes