PROTEIN TRAFFICKING. Dr. SARRAY Sameh, Ph.D

Size: px

Start display at page:

Download "PROTEIN TRAFFICKING. Dr. SARRAY Sameh, Ph.D"

Bryan Day
5 years ago
Views:

1 PROTEIN TRAFFICKING Dr. SARRAY Sameh, Ph.D

2 Overview Proteins are synthesized either on free ribosomes or on ribosomes bound to endoplasmic reticulum (RER). The synthesis of nuclear, mitochondrial and peroxisomal proteins occurs in the cytoplasm on free ribosomes Proteins that will eventually be associated with membranes (the endoplasmic reticulum, the Golgi complex and the plasma membrane) or secreted from the cell are synthesized on ribosomes that are bound to the endoplasmic reticulum, generating the rough ER (RER).

Trafficking of proteins synthesized bound to ER At the beginning of the translation process, the new protein have a specific sequence (N-terminal) called signal sequence (N-terminal) Signal sequence

3 Trafficking of proteins synthesized bound to ER At the beginning of the translation process, the new protein have a specific sequence (N-terminal) called signal sequence (N-terminal) Signal sequence is recognized and bound to the Signal recognition particle (SRP) which will direct the ribosome to the ER. The cytoplasmic side of ER contain receptors (SRP receptors) that specifically bind to SRP recruiting by that the entire ribosome to the ER SRP receptor is anchored to the translocation complex which is a kind of pore through it the protein pass into the lumen of ER as translation and elongation proceed. As translation ended, the ribosomes dissociate from the ER, the signal sequence is cleaved into the lumen and the protein makes its way to the correct membrane

4 Protein modifications in the lumen of ER while it is being translated the new proteins will be modified: co-translational process. Cytosol In the lumen of the ER, most of the proteins will be glycosylated: A sugar will be transferred from the membrane lipid (dolichol: oligosaccharide) to the new protein. These new proteins contains either two of three amino acids (Asn-X-Ser and Asn-X-Thr) and the sugar will be fixed to the amide group of Asn: N-glycosylation. The O-linked glycosylation occurs in the Golgi: carbohydrates are attached to OH groups of Ser or Thr amino acids within Asn-X-Ser/Thr

The ER membrane surround the nascent protein, buds off from the ER forming a transport vesicle.

5 Proteins trafficking to Golgi Some new proteins are used in ER, but most others start to move to Golgi through spaces within ER known as transitional element. The ER membrane surround the nascent protein, buds off from the ER forming a transport vesicle.. The transport vesicle transfer the new protein to Golgi complex, where it fuses with it This fusion is mediated by proteins called v-snare, located on the surface of the vesicles

6 Golgi complex The proteins from ER are sent to Golgi. It is the sorting organelle of the cell: Post office of the cell!! As the proteins moved through the Golgi stacks, they are modified by specific Golgi enzyme. These modifications are important because they provide the signal that determine the final destination of the protein. e.g: Proteins destined to work in lysosomes are phophorylated at mannose residues added to the protein, forming a mannose-6-p tag on lysosomal protein TGN (trans Golgi network) is the final sorting and packing region of Golgi: Some new proteins will remain in Golgi and function in processing of new transported proteins, others, are send onward to lysosomes or to the outside of the cell. How the proteins move through the Golgi?

7 Lysosomes - Lysosomes are membrane enclosed organelle with acidic ph that contain enzymes called acid hydrolyase -Acid hydrolyase precursors are tagged by mannose-6-p in Golgi as marker showing their destination (lysosome) which are recognized by receptors for mannose 6P in certain regions of Trans Golgi Network coated by clathrin protein. - New Mannose-6-P containing-proteins bind their receptors and the TGN membrane begins to bud off to enclose the new lysosomal proteins in a transport vesicle -Vesicles (bound for lysosomes) fuse with endosomes (transport vesicles from the plasma membrane created by endocytosis). The clathrin coat is lost - The ph is reduced within the precurssor lysosome by the pumping in of protons (H+) and the new protein dissociate from the M-6-P receptor and become functional enzymes - The receptors are recycled back to TGN.

Secretion from the cell The are two other important processes for the transportation of substances after being sorted by the TGN : the protein is secreted outside the cell These processes are called

9 Secretion from the cell The are two other important processes for the transportation of substances after being sorted by the TGN : the protein is secreted outside the cell These processes are called constitutive and regulated secretion. The constitutive secretion involves proteins that are important in maintaining cells and tissues, are constantly secreted [e.g., serum albumin and extracellular matrix (ECM)]. The regulated secretion occurs only when an external stimulus has been received, the vesicle containing the secretion and cell membrane fuse on receiving the stimulus, and the secretion is released outside the cell. This regulatory secretion pathway is used to secrete substances such as digestive enzymes and hormones.

10 Trafficking of proteins synthesized in free ribosomes Proteins destined to remain in the cell and function in the cytosol, nucleus, mitochondria or peroxisomes are synthesized on free ribosomes. Ribosomes synthesizing these proteins are free, because the protein lacks the N-terminal leading (signal) sequence that causes a ribosome to bind ER. However, proteins are still tagged by special structures that direct them to organelle, otherwise they will remain nonfunctional in the cytosol.

11 Cytoplasmic proteins Are the proteins which remains in the cytosol outside organelles They Lack leading signal and possess no structural feature that takes them to organelles Examples: enzymes of some pathways of CHO metabolism e.g. glycolysis and glycogen metabolism

12 Nuclear proteins Most of these proteins that gain access to the nucleus are tagged by a nuclear localization signal (NLS)) They bind strongly to improtin, a protein that facilitate their nuclear entry through a nuclear pore. Then importin dissociate (a GTPdependent process) and the new protein has reached its destination.

Mitochondrial protein The proteins are maintained in an unfolded form prior to entry in the mitochondria by the binding of chaperone proteins (that need ATP for their function).

13 Mitochondrial protein The proteins are maintained in an unfolded form prior to entry in the mitochondria by the binding of chaperone proteins (that need ATP for their function). These proteins are tagged by signal sequence called: import sequence (Nterminal) which bind to receptor located in Translocase of Outer Mitochondrial membrane (TOM) complex Once it reaches in the intermembrane space, the targeting signals binds to translocase of Inner Mitochondrial Membrane (TIM)TIM complex and the polypepetide chain enters the matrix.

14 Transport into peroxisomes Peroxisome house a variety of lipid oxidation reactions. Fully functional, fully folded protein is transported! The proteins contain C-terminal tripeptide (SKL: Ser-Lys-Leu) acting as peroxisomal targeting signal (PTS). PTS is not cleaved after internalization Similar to SRP and SRP receptor transport pathway.

15 Diseases due to defective protein targeting 1. Zellweger Syndrome 2. Primary hyperoxaluria 3. Familial hypercholestrolemia 4. Cystic fibrosis 5. Inclusion cell disease

16 The End!

Molecular Cell Biology Problem Drill 16: Intracellular Compartment and Protein Sorting

Molecular Cell Biology Problem Drill 16: Intracellular Compartment and Protein Sorting Question No. 1 of 10 Question 1. Which of the following statements about the nucleus is correct? Question #01 A. The