1. Intracellular Compartments and Transport
1- PROTEIN SORTING

2. Introduction
Organelle growth requires a supply of new lipids and proteins
Even in cells that are not dividing, proteins are being produced continually.
These newly synthesized proteins must be accurately delivered to their appropriate
Directing newly made proteins to their correct organelle is therefore necessary for a cell to be able to grow, divide, and function
For some organelles, including the mitochondria, chloroplasts, and the interior of the nucleus, proteins are delivered directly from the cytosol.
For others, including the Golgi apparatus, lysosomes, endosomes, and the nuclear membranes, proteins and lipids are delivered indirectly via the ER, which is itself a major site of lipid and protein synthesis.
Proteins enter the ER directly from the cytosol: some are retained there, but most are transported by vesicles to the Golgi apparatus and then onward to other organelles or the plasma membrane

3. The synthesis of virtually all proteins in the cell begins on ribosomes in the cytosol.
most mitochondrial and chloroplast proteins, however, are made in the cytosol.
few mitochondrial and chloroplast proteins that are synthesized on ribosomes inside these organelles;
The fate of any protein molecule synthesized in the cytosol depends on its amino acid sequence, which can contain a sorting signal that directs the protein to the organelle in which it is required.
Proteins that lack such signals remain as permanent residents in the cytosol; those that possess a sorting signal move from the cytosol to the appropriate organelle.
Different sorting signals direct proteins into the nucleus, mitochondria, chloroplasts (in plants), peroxisomes, and the ER.

4. Proteins Are Imported into Organelles by Three Mechanisms
Membrane-enclosed organelles import proteins by one of three mechanisms.
All of these processes require energy. the protein remains folded during the transport steps in mechanisms 1 and 3 but usually has to be unfolded in mechanism 2.

5. Proteins Are Imported into Organelles by Three Mechanisms
1-Proteins moving from the cytosol into the nucleus are transported through the nuclear pores that penetrate the inner and outer nuclear membranes. The pores function as selective gates that actively transport specific macromolecules but also allow free diffusion of smaller molecules. 2-Proteins moving from the cytosol into the ER, mitochondria, or chloroplasts are transported across the organelle membrane by protein translocators located in the membrane. Unlike transport through nuclear pores, the transported protein molecule must usually unfold in order to snake through the membrane 3-Proteins moving from the ER and from one compartment of the endomembrane system to another are transported by a mechanism that is fundamentally different from the other two. These proteins are carried by transport vesicles. The vesicles subsequently discharge the protein by fusing with its membrane.

6. Signal Sequences Direct Proteins to the Correct Compartment
The typical sorting signal on proteins is a continuous stretch of amino acid sequence, typically 15–60 amino acids long. This Signal sequences direct proteins to the correct organelle.
proteins destined for the ER possess an N-terminal signal sequence that directs them to that organelle, whereas those destined to remain in the cytosol lack this sequence.
if the signal sequence is removed from the ER protein and attached to the cytosolic protein, the proteins end up in an abnormal location in the cell. Such experiments indicate that the ER signal sequence is both necessary and sufficient to direct a protein to the ER.
The transfer process is halted by an additional sequence of hydrophobic amino acids, called a stoptransfer sequence

7. Proteins Enter the Nucleus Through Nuclear Pores
The outer nuclear membrane is continuous with the ER. The double membrane of the nuclear envelope is penetrated by nuclear pores.

8. A nuclear pore is composed of about 30 different proteins
A nuclear pore is composed of about 30 different proteins. Each pore contains water-filled passages through which small water-soluble molecules can pass freely and nonselectively between the nucleus and the cytosol. Meshwork fills the center of the channel, preventing the passage of large molecules but allowing smaller molecules to slip through. Larger molecules (such as RNAs and proteins) and macromolecular complexes must display an appropriate sorting signal to pass through the nuclear pore. The signal sequence that directs a protein from the cytosol into the nucleus, called a nuclear localization signal. Cytosolic proteins called nuclear transport receptors bind to the nuclear localization signal on newly synthesized proteins destined for the nucleus. These receptors help direct the new protein to a nuclear pore by interacting with the tentacle-like fibrils that extend from the rim of the pore.

9. ER
The endoplasmic reticulum (ER) is the most extensive membrane system in a eucaryotic cell.
it serves as an entry point for proteins designed for other organelles, as well as for the ER itself.
Proteins destined for the Golgi apparatus, endosomes, and lysosomes, as well as proteins destined for the cell surface, all first enter the ER from the cytosol.
After synthesis in ER Protein will be carried by transport vesicles from organelle to organelle and, in some cases, from organelle to the plasma membrane or to the exterior of the cell.

10. Rough Endoplasmic Reticulum
The rough endoplasmic reticulum manufactures membranes and secretory proteins. The ribosomes attached to the rough ER synthesize proteins by the process of translation. In certain leukocytes (white blood cells), the rough ER produces antibodies. In pancreatic cells, the rough ER produces insulin. The rough and smooth ER are usually interconnected and the proteins and membranes made by the rough ER move into the smooth ER to be transferred to other locations. Some proteins are sent to the Golgi apparatus by special transport vesicles. After the proteins have been modified in the Golgi, they are transported to their proper destinations.
Smooth Endoplasmic Reticulum
The smooth ER has a wide range of functions including carbohydrate and lipid synthesis. Lipids such as phospholipids and cholesterol are necessary for the construction of cell membranes. Smooth ER also serves as a transitional area for vesicles that transport ER products to various destinations. In liver cells the smooth ER produces enzymes that help to detoxify certain compounds. In muscles the smooth ER assists in the contraction of muscle cells, and in brain cells it synthesizes male and female hormones

11. Two kinds of proteins are transferred from the cytosol to the ER:
Water-soluble proteins are completely translocated across the ER membrane and are released into the ER lumen;
Designed either for secretion or for the lumen of an organelle;
Transmembrane proteins are only partly translocated across the ER membrane and become embedded in it.
Designed to reside in either the ER membrane, the membrane of another organelle, or the plasma membrane.