The endoplasmic reticulum and the Golgi apparatus permit

The endoplasmic reticulum and the Golgi apparatus permit protein construction and transport in a cell. 11/20 How are cells are divided into organelles for localized specialization within a cell? What pathways do proteins take post-translation from m. RNA to ribosomes in the RER and cytosol? How/where are proteins modified in a cell? SER stores Ca++ and modify proteins. How does a cell use the SER to create differences in the PL content of its bilayers? How does the Golgi Apparatus participate in exocytosis and endocytosis? How does clathrin help mediate receptor-mediated endocytosis?

Cellular compartmentalization allows for local specialization in eukaryotic cells. – Who we have seen: ¬ Nucleus: info stored here ¬ Mitochondria: power plant ¬ Chloroplast: powerplant – Who we will see today: ¬ Endoplasmic reticulum: make protein here – Also: store calcium, metabolize drugs, modify glycogen ¬ Golgi Apparatus: modify/export protein here – Modify proteins for export here ¬ Vesicles and Endosomes: bulk entry/exit points into cell – Who we won’t see much of…. ¬ Lysosome: digest material inside cell ¬ Peroxisome: digest long chain fatty acids and bacteria

What pathways do newly transcribed proteins take when going to their ultimate destinations? ER folds have HUGE surface when spread out! OPTION ONE: 1) m. RNA finds a ribosome on ER membrane: Protein created 2) Rough E. R. : Protein sent into lumen 3) Golgi Apparatus: Protein to 3 destinations: a) Plasma membrane b) Lysosome c) Secretory Vesicles Constituitive/Regulated ¬ OPTION TWO: 1) m. RNA find ribosome: Protein created 2) Ribosome stays free in cytosol Protein released to cytosol 3) Protein to intracellular destination Huge holes in target let protein enter: a) Nucleus: b) Mitochondria c) Chloroplast d) Peroxisome “Porins” permit entry into

Basic Pathway: nucleus m. RNA Ribosome: ER Cis. Golgi. Cisternae Trans-Golgi Option Two: Cytosol Pores Target Organelle

Many proteins are modified when they are put into the ER lumen after translation! ¬ Sequences are clipped off: Insulin ¬ Disulfide bonds may be added: Insulin ¬ Carbohydrates may be added: N-CAMS ¬ Amino Acids may be modified to modify protein conformation: Hydroxylysine/Hydroxyproline ¬ Chaperones may fold nascent protein into precise conformations: Hemoglobin ¬ Additional Modifications often take place in the Golgi Apparatus

The smooth ER is devoid of ribosomes and helps modify proteins and metabolize drugs/toxins. Cytochrome P-450 reduces target molecules with electrons taken from NADPH Hepatocytes are really good at making NADPH P-450 destroys carcinogens/toxins System increases solubility/removal of metabolites Water soluble toxins no longer reabsorbed BASIC RXN: R-H+NADPH+H++O 2 R-OH+NADP+H 2 O Cells make more P-450 if stimulus continues! This is called enzyme or P 450 induction! This cellular event explains why resistances to some drugs develop over time! AIDS, Addiction, Alcoholism, Toxic Resistance and why some drugs just stop working

The smooth ER also helps with calcium, lipid transport, and carbohydrate metabolism. Calcium can be stored in the SER – Some SER rich in IP 3 -regulated channels/Ca-ATPase – Exit from SER in response to signal transduction cascade – Special muscle SER is called “Sarcoplasmic Reticulum” – Tons of sarcoplasmic reticulum in cardiac/skeletal myocytes – Keep cytosolic Ca in micromolar concentrations at rest ¬ SER enzymes regulate glycogen metabolism c. AMP-dependent glycogen phosphorylase here! Glycogen G-1 -P G-6 -P Why does muscle SER lack glucose-6 -phosphorylase? Why does hepatocyte SER have glucose-6 -phosphorylase?

Phospholipids are not evenly stacked in the PL bilayers of different cells/organelles

How do cells create lipid bilayers with differing ratios of phospholipids? Membrane Asymetry is critical for cellular/organelle functions! ¬ Lipid Bilayer content determines function! – Ratio of different lipid types is important in disease! ¬ 3 -Glycerol kinase creates Phospholipids ¬ Lipids easily flip PLs across bilayer halves ¬ Lipids don’t easily flip from bilayer. A to bilayer. B ¬ Phospholipid Translocate (“Flippase”) allows PL to be moved from SER into target bilayers – Targets: Plasma membrane and nucleus ¬ PL Exchange protein moves specific PL to membranes of organelles – Targets: mitochondria, chloroplasts and peroxisomes.

The Golgi Apparatus is a massive organelle that modifies and targets protein for export: SER Cic. Golgi>Medial. Cisterna>Trans. Golgi>Export

Most proteins move directionally towards the plasma membrane (Anterograde Tx)for secretion, sometimes proteins move from PM to ER (Retrograde Tx).

Exocytosis : fusion of secretory vesicles with PM Constituitive: happens at same rate all the time Regulated: happens in response to stimulus (Ca) How does this explain how glucose transporters (GLUT) in a vesicle reach the PM following a meal and insulin release? Insulin Stimulate. Tyrosine. Kinase activated Phospholipase Lipase releases IP 3 into cytosol IP 3 opens SER Ca++-channels Ca++ initiates V-fusion with PM Vesicle GLUT now on PM Glucose can enter the cell!

Receptor-mediated endocytosis permits a cell to pull large particles inside after binding to receptors that line clathrin coated pits!

Comments on Endocytosis into coated vesicles: ¬ Clathrin and receptors are typically recycled back to the PM! – Clathrin has a triskelion-like shape! – Provides structure to pit and attachment point for intracellular scaffolding ¬ Endocytotic vesicles may fuse with lysosomes to modify/digest vesicle contents! – Lysosomes are rich in acids and digestive enzymes ¬ Contents in lysosome removed by diffusion, remnants removed by exocytosis! ¬ Many bacteria, protozoan paratites and viral particles get access to a cell by locking to PM receptors! ¬ Drugs may prevent of docking/cell infection!