Cell Biology - Intracellular Transport PDF

Intracellular components and transport – WK1 Organelles and functional specialisation Transport across membranes Simple Diffusion: permeable to the solute Osmosis: is typical cell membrane Selective Transport across membranes Cells receive/send messages which aids hormonal signals, init...

Intracellular components and transport – WK1 Organelles and functional specialisation Transport across membranes Simple Diffusion: permeable to the solute Osmosis: is typical cell membrane Selective Transport across membranes Cells receive/send messages which aids hormonal signals, initiating actions in sensing external stimuli Ensuring essential molecules the cells, the metabolic intermediates remains in cell – waste compounds leave cell which is constant internal environment The transport of ions vital for pH and osmotic pressure regulation Endomembrane system – intracellular transport  The molecules of the solutes that are transported across the membranes of eukaryotic cells Enlargement of pumping across membranes during electron transport – the use of the results electrochemical potential of drive ATP synthesis in these organelles. How do molecules take up and transport and release molecules? Membrane budding and fusion underlie several important biological processes Exocytosis such as in the release of neurotransmitters Receptor mediated Endocytosis Exocytosis Fusion of secretory vesicles with the plasma membrane Results in discharge of vesicles content into extracellular matrix Incorporation – new proteins into lipids into plasma membrane Endocytosis Material to be internalized is surrounded by an area of cell membrane Which then buds off inside the cell to form a vesicle containing the ingested material Phagocytosis 1. The cell engulfs the particle e.g. bacteria 2. The bacteria binds to the cell surface – stimulates the extension of a pseudopodium which engulfs bacteria 3. Fusion of pseudopodium membranes the results of formation of a large intracellular vesicle = phagosome 4. Phagosome fuses with lysosome forms a phagolysosome – the contents are digested 5. Predominant in Amoeba and immune cells such as macrophages Pinocytosis 1. Uptake of fluids of macromolecules in small vesicles 2. Soluble material in the extracellular fluid are taken up incorporated into vesicles for digestion 3. The digested solutes released into cytosol as all solutes are taken via method of endocytosis – unspecific Receptor-mediated endocytosis 1. Selective uptake of material by animal cells 2. Macromolecules bind to receptor the cell membrane 3. Enters the Clathrin – coated vesicle 4. These Vesicles fuse with early endosomes – their content is sorted for transport to lysosomes – recycling to plasma membranes like cholesterol uptake Protein Sorting: The cellular compartment contains unique set proteins These are made in Cytosol And they are selectively transferred to their specalised compartment All proteins that pass through the Golgi apparatus – except those they are retained there as permanent residents and sorted in trans Golgi network by the specific signals according to final destination Depends on… - Signal sequence in amino acid sequence Receptors for the signal sequences Translocation channels available Energy that drives transfer across the membrane Protein Sorting Pathways: Non-secretory pathway: this is where targeting proteins to nucleus peroxisomes, cytoplasm and mitochondria ( ALL OF IT ) Secretory Pathway: the endoplasmic reticulum, the proteins are transported to the vesicles to the Golgi apparatus and this where they are further processed and sorted for transport to lysosomes, the plasma membrane or secretion from the cell. Nonsecretory Pathway Nucleus: NLS – nuclear localization signal where there are short peptide sequences responsible for direct import of proteins into nucleus One or more short sequences of positively charged lysines or arginines exposed on the protein surface. Mitochondria and chloroplast Double membranes Contain their own genomes but still import most proteins from the cytosol. Proteins are unfolded as they are transported and the N terminal signal sequence is removed once translocation is complete. Proteins destined for the matrix are translocated simultaneously. Chaperone proteins help to pull proteins across the membranes and refold the protein once inside. Transport to particular organelle sites is directed by further sorting signals. Mitochondrial Matrix Protein Peroxisome Protein : All peroxisome made in cytosol and imported post translationally Peroxisomes preformed oxidative reactions, isolate and breakdown harmful hydrogen peroxide ATP derives from trans locational matrix proteins across membrane Many Matrix Proteins fold the cytosol and cross the membrane their folded conformation, their targeting sequences are not cleaved after import. Peroxisomal membrane and matrix proteins contain different sequences there incorporated by different pathways. Secretory Pathway: The endoplasmic reticulum which the Golgi apparatus and the vesicles that travel in between them as well as the cell membrane and lysosomes. It is secretory of being pathway which the cell secretes proteins in the extracellular environment Most cellular transmembrane proteins use this pathway as a final destination. Cytosol and the ‘lumen’ they are different chemical environments and they never mix Cytosol – is reductive the ER/Golgi and extracellular environment are oxidative Co – Translation of Translocation Some proteins headed for the ER lumen enter the ER as they are being made (during translation) ER lumen proteins are targeted for the ER by co-translational translocation The synthesis of co-translationally translocated proteins begins on free ribosomes. These proteins have a unique signal sequence that is near the N-terminus A protein-RNA complex called signal recognition particle (SRP) recognizes and binds to the signal sequence and to the ribosome, halting translation Post- translational translation While most ER lumen proteins are targeted for the ER by cotranslational translocation, some are made on free ribosome then translocated into the ER New proteins move into the ER after their translation Protein glycosylation: Other chemical modifications occur in the ER lumen, such as the addition of oligosaccharides (glycosylation). External membrane proteins are glycosylated this way. Oligosaccharide transferase, a part of the translocon complex adds glycosyl groups to asparagines in the nascent protein Glycosylation is an important and highly regulated mechanism of secondary protein processing within cells It plays a critical role in determining protein structure, function and stability Important in protein folding Protein folding and processing in the ER: Polypeptides must assume the correct folding pattern in order to function properly, mediated by chaperones (they also are proteins--chaperones are abundant in the ER lumen). Chaperones bind to polypeptides destined for mitochondria then release them as they pass through the mitochondrial membranes. Chaperones on the inside of mitochondria bind until these polypeptides have completely entered. A completed polypeptide will assume the correct folding pattern spontaneously, but some could assume incorrect formation or it could aggregate with other partially made polypeptides before translation is complete To prevent this, chaperones in the ER (and cytosol) bind to the nascent polypeptide and keep it from interacting with anything until the polypeptide is completely synthesized. Protein Folding and processing in the ER: Calreticulin a calcium binding integral protein is an ER resident protein Incorrectly folded proteins can be re –glycosylated to gain conformation Misfolded proteins retained in the ER are retro-translocated to the cytosol to be degraded by the proteasome This mechanism, known as ER associated degradation (ERAD) ER transmembrane proteins During transport, the topology of a TMP is preserved; the same segments of the protein, for instance, always face the cytosol. Thus the orientation of these membrane proteins in their final sites is established during biosynthesis on the ER membrane. Single-pass and multipass TMP. ER transmembrane proteins: Topologies of some integral membrane proteins synthesized on the rough ER. Each protein has a unique orientation with respect to the membrane’s phospholipid layer. Single pass transmembrane proteins A single-pass TMP only passes through the phospholipid bilayer once characterised by N-terminal motif on the exoplasmic face and their hydrophilic Cterminal segment on the cytosolic face. Multipass Transmembrane proteins: A multipass TMP only passes through the phospholipid bilayer multiple times characterised by N-terminal motif on the cytosolic face. ER and the Golgi apparatus: Transport from the Golgi apparatus: The trans-Golgi network is a highly dynamic series of interconnected tubules and vesicles at the trans face of the Golgi stack. Functions in the processing and sorting of glycoproteins and glycolipids at the interface of the biosynthetic and endosomal pathways. A major secretory pathway sorting station Directs newly synthesized proteins to different subcellular destinations Receives extracellular materials and recycled molecules from endocytic compartment Retrieval of resident ER proteins: ER resident proteins are retained in the ER after folding Retrieval signals allow for retrieval from the Golgi apparatus by ER retention receptors Signal is recognised by KDELRs either when bound to unfolded client proteins or in free form KDEL is a target peptide sequence in mammals and plants located on the Cterminal end of the amino acid structure of a protein. The KDEL sequence prevents a protein from being secreted from the ER and facilitates its return if it is accidentally exported KDEL (Lys-Asp-Glu-Leu) ERGIC – ER-Golgi intermediate compartment Examples of ER resident proteins are BiP and PDI