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 mRNA 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) mRNA 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 membraneb) Lysosomec) Secretory Vesicles

Constituitive/Regulated

OPTION TWO:1) mRNA find ribosome:

Protein created2) Ribosome stays free in cytosol

Protein released to cytosol3) Protein to intracellular destinationHuge holes in target let protein enter:a) Nucleus: b) Mitochondriac) Chloroplastd) Peroxisome“Porins” permit entry into organelle

Basic Pathway: nucleus mRNA Ribosome:ERCisGolgiGolgiCisternaeTrans-GolgiOption Two: CytosolPoresTarget 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 NADPHP-450 destroys carcinogens/toxinsSystem increases solubility/removal of metabolitesWater soluble toxins no longer reabsorbed BASIC RXN:R-H+NADPH+H++O2R-OH+NADP+H2O

Cells make more P-450 if stimulus continues!This is called enzyme or P450 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 IP3-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 metabolismcAMP-dependent glycogen phosphorylase here!

GlycogenG-1-PG-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 bilayerA to bilayerB

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: SERCicGolgi>MedialCisterna>TransGolgi>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 timeRegulated: 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 StimulateTyrosineKinaseKinase activated Phospholipase

Lipase releases IP3 into cytosol

IP3 opens SER Ca++-channels

Ca++ initiates V-fusion with PM

Vesicle GLUT now on PMGlucose 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!