M2C: Membrane Biogenesis And Protein Targeting

Front 1

Cell membrane in temporal continuity with endoplasmic reticulum (ER)

Back 1

Rough and smooth ER, nuclear membranes, golgi apparatus, secretory vesicles, plasma membrane, lysosomes, and endosomes. Synthesis of both lipids and proteins occur in the ER.

Front 2

Cell membrane that is temorally distinct

Back 2

Inner and outer mitochondrial membranes and peroxizomes.

Front 3

Synthesis of membrane lipids

Back 3

Mostly in the ER. Enzymes on the cytoplasmic face of the ER and new lipids on the cytoplasmic leaflet of the ER bilayer. Flipasses (lipid translocases) transport new lipids from the cytoplasmic leaflet to the bilayer of the luminal leaflet of the bilayer. Flipasses have higher affinities for certain lipids which leads to lipid asymmetry in the cell membranes.

Front 4

Lipid transport from ER to membrane

Back 4

Vessicles move new lipids to Golgi apparatus. Here glycosylation of some lipids forms glycolipids. From golgi, vessicles bleb off and do vesicle fusion with other membranes (plasma membrane and lysosomes) to add new lipids.

Front 5

Cardiolipin

Back 5

Much of mitochondrial and perioxisome lipids are formed in the mitochondia.

Front 6

Phospolipid exchange proteins

Back 6

Soluble proteins that transport membrane lipids from ER to mitochondria and perioxisomes.

Front 7

Protein Synthesis

Back 7

All formed in ribosomes. Translate the information encoded in a sequence of bases in mRNA in to a linear sequence of aa that make up specific proteins.

Front 8

Ribosomes

Back 8

Factories for making proteins from an mRNA template. Composed of two subunits. Each is made up of RNA and many subunits. All synthesis begins on free cytoplasmic ribosomes. Depending on type of protein being made, some ribosomes will stay as free cytoplasmic while others will associate with the ER giving rise to rough ER (RER).

Front 9

Free Cytoplasmic Ribosomes

Back 9

Include all cytosolic proteins, cytoskeletal proteins, nuclear proteins (not nuclear membrane, but within the nucleus), peripheral membrane proteins, and proteins in or within the membranes of peroxisomes and mitochondria.

Front 10

Membrane-bound ribosomes

Back 10

Category 1 proteins. Proteins destined for secretion.

Front 11

Synthesis of membrane and secreted proteins

Back 11

Starts in free cytoplasmic ribosomes. First part of protein, "signal sequence" peptide. This is bound to the signal recognition particle (SRP). This binding halts translation. SRP binds to the SRP receptor (SRPR) in the membrane of the RER. Translation starts again. The signal sequence then binds to the signal sequence binding protein in the RER. This forms part of the protein translocator channel in the RER membrane. N-(asparagine)-linked oligosaccharides may be added. The SRP is released from the newly synthesized protein which is passed through the translocator channel into the lumen(cistema) of the RER. Chaperone proteins prevent misfolding as new protein moves through RER lumen. Signal pepsidase cleaves off signal peptide. Protein is then free within ER lumen and toplogically outside cell.

Front 12

Synthesis of membrane protein that spans the membrane once

Back 12

Same as general synthesis of membrane and secreted proteins. Follows same protein translocator channel until reaches a "stop transfer sequence" within the polypeptide sequence. This is a hydrophobic stretch of aa destined to become the transmembrane domain. Continued protein synthesis will generate the cytoplasmic domain. Finally the signal sequence is cleaved off by the signal peptidase.

Front 13

One or many ribosomes?

Back 13

The larger the protein being synthesized, the more ribosomes may be bound. More than one ribosome on the same protein is called a polyribosome or polysome.

Front 14

Cotranslational Import

Back 14

The insertion of a membrane protein into the RER membrane during translation.

Front 15

Protein modification in the Lumen of the ER

Back 15

1. Correct folding of the protein
2. Formation of disulfide bonds in the RER lumen
3. Glycosylation begins in the lumen of the RER.

Front 16

Synthesis of a membrane protein that spans the membrane multiple times

Back 16

Same as with one membrane span, but after first stop transfer sequence and the synthesis of some cytoplasmic sequence, there will be another signal sequence or start transfer sequence. These alternating start and stop transfer sequences give rise to hydrophobic aa regions, each of which can span the membrane. This allows a single membrane protein to span the membrane multiple times.

Front 17

Membrane Trafficking

Back 17

How new proteins get from site of synthesis in RER to final destination. After formation of the protein, membrane vessicles bud off of the ER and fuse with the forming face (Cis face) of the Golgi. Fusion maintains the orientation of the membrane and empties the contents into the lumen of the Golgi.

Front 18

Golgi Apparatus

Back 18

Complex arrangement of stacks of cisternae. Modifies proteins and lipids synthesized by ER. Sorts various components. Major site of glycosylation of membrane and secreted proteins. Sulfation and phosphorylation of proteins occurs here as well. Some proteolytic processing.

Front 19

Transport through Golgi

Back 19

Golgi made of successive compartments known as cis, medial, trans cisternae, and finally the trans-golgi network (TGN). Material is passed from one cisternae to the next by the "budding off" of vessicles and the fusion of vessicle membrane with the next cisternae. Cystolic and integral membrane proteins known as SNARES promote fusion of transfer vesicles with target membranes.

Front 20

Sorting of proteins

Back 20

Occurs in the trans-golgi network. Different transport vesicles bud off from the last part of the golgi heading to final destination of protein (plasma membrane, lysosomes, etc. Proteins heading to lysosomes are sequestered from those going to plasma membrane by mannose-6-phospate. Those not bound by mannose-6-P head to the plasma membrane where vesicles fuse and release protein out of the cell.

Front 21

I-Cell Disease

Back 21

Mannose-6-Phosphate is not added to proteins destined for lysosomes. Causes failure of lysosome function. Cellular digestion ceases. Lysosomes swell.

Front 22

Tay-Sachs Disease

Back 22

Defect in one lysosome enzyme that breaks down a certain glycolipid. Fatal at early age.

Front 23

Secretory Vesicles

Back 23

1. Constitutive: vesicles are continually transported to the plasma membrane for secretion. Ex. Plasma cells continually release immunoglobulin proteins.
2. Regulated: Vesicles are stored until cell is signaled to release them. Under hormonal or neuronal regulation. Important that only happen at certain times like with digestive proteins or synaptic vesicles by nerves with synaptic transmission.

Front 24

Membrane protein display in different cells

Back 24

Many cells have polarized organization and display different proteins at different regions of their plasma membrane. Ex. epithelial cells have an apical surface that is distinct in its membrane protein composition from its basolateral surface. Different vesicals from the golgi are specified and destined to these different areas. Diffusion of components between these two regions is prevented by tight junctions.

Front 25

Hepatitis viruses and membranes

Back 25

Hepatitis viruses infect hepatocytes. Hepatitis A is targeted to the apical surface of the liver cells and is consequently shed to the bile canaliculi. Contents of this ducts (bile) drain into the gall bladder and then intestine. Therefore transmitted through fecal matter. In comparison, Hepatitis B is targeted to the basolateral surface of the liver cells. This virus enters the bloodstream instead of the bile canaliculi and therefore is transmitted through infected blood.

Front 26

Proteins needed in the RER

Back 26

Contain a retention signal that either blocks protein passage to the golgi or is involved in their segregation and transport back to the RER.

Front 27

Synthesis of Mitochondrial and Proxisome Proteins

Back 27

Few are synthesized in the mitochondria on mitochondrial ribosomes. All the rest are synthesized on free cytoplasmic ribosomes. These proteins have signal sequences that are recognized by receptors on the mitochondria and peroxisome membranes. If they contain a stop transfer sequence, synthesis will cease and they will become membrane proteins on their organelle, if not, they will be transported into the organelle. Receptors on mitochondria occur at contact sites where inner and outer membranes come into contact. For this, transport will occur across both membranes transporting protein into mitochondrial matrix. Additional signal sequences direct proteins to wherever they need to be in the mitochondria. Additional stop sequences allow proteins to be integrated into inner membrane if that is where they are destined.

Front 28

Post-tranlational Import

Back 28

The insertion of peroxisomal and mitochondrial membrane proteins into membranes.

Front 29

Synthesis of nuclear proteins

Back 29

Occurs in the RER. Outer nuclear membrane is continuous with the RER and have ribosomes on surface. Non-membrane proteins in the nucleus are synthesized by free cytoplasmic ribosomes. These proteins contain nuclear targeting sequences and enter nucleus through nuclear pores.