PROTEIN TRAFFICKING IN LYSOSOMES AND

VESICULAT TRANSPORT

NAME: Sheryl BhatnagarRoll no.: 2047

Course: Botany(h) 3rd yr

Contents • Introduction • Translocation pathways• Lysosomes• Modifications of lysosomal proteins• Vesicular transport• Coated vesicles• Clathrin coated vesicles• Formation of coated vesicles• Vesicle fusion

Protein trafficking• Protein trafficking is the mechanism by which a cell

transports proteins to the appropriate positions in the cell or outside of it.

• Both in prokaryotes and eukaryotes, newly synthesized proteins must be delivered to a specific subcellular location or exported from the cell for correct activity. This phenomenon is called protein trafficking.

• This delivery process is carried out based on information contained in the protein itself.

• Correct trafficking is crucial for the cell; errors can lead to diseases.

• In 1970, Günter Blobel conducted experiments on the translocation of proteins across membranes.

• He was awarded the 1999 Nobel Prize for his findings. He discovered that many proteins have a signal sequence, that is, a short amino acid sequence at one end that functions like a postal code for the target organelle.

Translocation PathwaysCO- TRANSLATIONAL PATHWAY POST-TRANSLATIONAL PATHWAY

• Transport occurs while the polypeptide chain is being synthesized on a membrane-bound ribosome.• Signal sequences are bound by signal recognition particle (SRP) .

• The polypeptide chain is completed in the cytoplasm before being transported into the endoplasmic reticulum.• Signal sequences recognized by receptors on translocon (not need SRP) .

Lysosomes • Lysosomes are spherical membranous

bags that contain enzymes (Acid Hydrolases).

• size varies from 0.1–1.2 μm.• The lysosomal enzyemes are capable of

digesting all varieties of biological molecules

Functions:I. Digestion of worn out or non

functional organelles.II. Metabolic functionsIII. Degradation of nonuseful tissue

Some important enzymes found within lysosomes include:

Lysosomal Enzymes Biological Molecules

Amylase Carbohydrates

Proteases Proteins

Nucleases Nucleic acids

Lipase Lipids

Phosphoric acid Monoesters

Modifications of Lysosomal Proteins

• The processing of N-linked oligosaccharides of lysosomal proteins differs from that of plasma membrane and secreted proteins.

• The lysosomal proteins are modified by mannose phosphorylation.• First, there is addition of N-acetylglucosamine phosphates to specific mannose residues, and

this happens probably while the protein is still in the cis Golgi network.• After this N-acetylglucosamine group is removed, leaving mannose-6-phosphate residues on

the N-linked oligosaccharide. • The phosphorylated mannose residues are specifically recognised by a mannose-6-phosphate

receptor in the trans Golgi network, which directs the transport of these proteins to lysosomes.

Vesicular transport• Proteins from the ER to the Golgi

apparatus and proteins to E.R and from golgi to cell organelles, for example, occurs in this way.

• transport intermediates— which may be small, spherical transport vesicles or larger, irregularly shaped organelle fragments—carry proteins from one compartment to another.

Coated vesiclesRole of the coat:

Components of the membrane (e.g. receptors) are concentrated into patches

Removal of coated surfaces and formation of vesicles

Types of coated vesicles: Clathrin-coated

vesicles

COPI-coated vesicles

COPII-coated vesicles

Clathrin-coated vesicle: Golgi – surface mamembrane transport

COPI- and COPII-coated vesicles:rER - Golgi transports

Clathrin-coat Triskelion zikk-zakk and globular structural elements

The Formation of coated vesicles• The formation of coated vesicles is regulated by small GTP-binding

proteins related to Ras and Ran.

• Two families of GTP-binding proteins play roles in transport vesicle budding: ADP-ribosylation factors (ARFs 1-3 & Sarl) and a large family of Rab proteins.

• These regulate adaptor proteins that interact directly with a vesicle coat protein.

• The binding of GTP-binding proteins and adaptor proteins establishes a "platform" on the membrane for a specific process, such as assembly and budding of a transport vesicle directed from the transitional ER to the Golgi or from the trans Golgi network to endosomes and lysosomes.

• Individual proteins in the complex (coat proteins, adaptor proteins, and GTP-binding proteins) may participate in assembly of transport vesicles directed elsewhere, or in vesicle,but each protein complex is apparently unique to a particular budding, transport, or fusion pathway.

Vesicle Fusion• The fusion of a transport vesicle with its target involves two types

of events.

• First, the transport vesicle must recognize the correct target membrane; for example, a vesicle carrying lysosomal enzymes has to deliver its cargo only to lysosomes.

• Second, the vesicle and target membranes must fuse, deliver- ing the contents of the vesicle to the target organelle.

• Analysis of the proteins involved in vesicle fusion in these systems led Rothman and his colleagues to pro- pose a general model, called the SNARE hypothesis, in which vesicle fusion is mediated by interactions between specific pairs of transmembrane proteins, called SNAREs, on the vesicle and target membranes (v-SNAREs and t-SNAREs, respectively).

• .

According to the hypothesis, the formation of complexes between v-SNAREs on the vesicle and t-SNAREs on the target membranes leads

to membrane fusion

Cont.• SNAREs are required for vesicle fusion with a target membrane and that

SNARE-SNARE pairing provides the energy to bring the two bilayers sufficiently close to destabilize them and result in fusion.

• However, the docking, tethering, and fusion of transport vesicles to specific target membranes appears to be mediated by a sequentially assembled protein complex much like that which led to transport vesicle budding.

• Members of the Rab family of small GTP-binding proteins play key roles in this docking of transport vesicles.

• Rab proteins, like the ARF family, participate in many of the vesicle budding and fusion reactions during vesicular transport.

• More than 60 different Rab proteins have been identified and shown to function in specific vesicle transport processes.

• They function in many steps of vesicle trafficking, including interacting with SNAREs to regulate and facilitate the formation of SNARE/SNARE complexes.