Endocytosis

Question 1

How are soluble lysosomal enzymes recognized for transport

Answer 1

They’re recognized by enzymes that add phosphate groups to the mannose sugars of n linked carbohydrate chains

The phophorylated mannose (mannose 6-phosphate) Residues acts as sorting signals which direct protiens to the lysosome

Question 2

What is the first step in targeting lysosomal enzymes to the lysosome

Answer 2

The mannose residues )the lysosomal enzymes) are phosphorylated in the cis golgi to make mannose 6-phosphate

Question 3

What is the second step in targeting lysosomal enzymes to the lysosome

Answer 3

The phosphorylated lysosomal enzymes are brought into a clathrin coated vesicle in the trans Golgi

This happens by the use of the mannose 6 phosphate receptor (MPR), the GGA adaptor, and the clathrin

Question 4

What is the MPR

Answer 4

It’s a transmembrane protien that recognizes and captures protiens that have the mannose 6 phosphate signal

Question 5

What is the GGA adaptor

What is the clathrin

Answer 5

It connects the clathrin to the MPRs

The coat protien that makes a structural scaffold of the vesicle

Question 6

For step two of targeting lysosomal enzymes to lysosomes, how is the clathrin coated vesicle formed

Answer 6

The g protien ARF 1 binds to the golgi membrane and begins formation of a budding vesicle and binding of the coat protiens

The GGA adaptor which has 3 domains binds to ARF 1 GTP, CLATHRIN, AND MPR

adaptor physically links two or more components

Question 7

What is step 3 and 4 and 5 of targeting lysosomal enzymes to lysosomes

Answer 7

3. Formation of the clathrin coated vesicles and separating from trans golgi
4. The MPRs separated from the lysosomal enzymes In the vesicle
5. MPRS are recycled back the trans golgi

Question 8

What is step 6 of targeting lysosomal enzymes to lysosomes

Answer 8

6. The after the receptors are recycled, the clathrin coat on the vesicle is disassembled

the lysosomal enzymes are sent to a sorting endo some then onto a lysosome

Question 9

What is step 7 of targeting lysosomal enzymes to lysosomes

Answer 9

The MPRs that were previously disassembled are also in the plasma membrane

They are there to capture any lysosomal enzymes that get secreted into the extracellular space

They then return them back to the lysosome

Question 10

If the proteins in the golgi need to leave but ARENT going through lysosomes what is the default way to go

Answer 10

The cisternae of the golgi moves to the TGN and fragments into vesicles and tubules

Constitutive secretion is the default (they go in a secretory granule)

Question 11

What do sorting signals do

Answer 11

They direct secretory protiens to the plasma membrane of polarized cells

Question 12

What are the two types of endocytosis (things into cell)

Answer 12

Bulk phase

Receptor mediated

Question 13

What is bulk phase endocytosis

Answer 13

Also called pinocytosis (cell drinking)

It does non specific uptake of extracellular fluids and any molecules in the fluid into the cell

Question 14

What is receptor mediated endocytosis

What things are borough I

Answer 14

Also called clathrin mediated

It’s when SPECIFIC molecules bind to the receptors on the extracellular surface of the plasma membrane (so they can then be brought into the cell)

Ex. Hormones, growth factors, certain nutrients

Question 15

What is each clathrin molecule called and made of

Answer 15

Called a triskelion

Made of three heavy chains and three light chains

Question 16

What does the AP2 complex do in clathrin (receptor) mediated endocytosis

Answer 16

In the cell, it links the cytoplasmic tails of the receptors in the plasma membrane to clathrin

So it goes receptors in cell membrane, adaptor on tail of receptor inside cytoplasm, clathrin on adaptor in cytoplasm

Question 17

What is dynamin and its role in clathrin coated vesicle separation

Answer 17

It’s a G protien that is required for the clathrin coated vesicle to bud from the membrane

The dynamin subunits polymerize and form a dynamin ring under the forming vesicle

GTP hydrolysis to gdp makes the dynamin ring move and cleave the vesicle

The dynamin ring is then diassembled

Question 18

What are the two outcomes of endocytosis

Answer 18

The recycling pathway

The degredation pathway

Question 19

What is the recycling pathway of endocytosis

Answer 19

First housekeeping receptors on the surface of the cell membrane mediate the uptake of things that will be used by the cell (cholesterol iron etc)

The receptors are moved to an early endosome to be sorted and they dissociate from the protiens due to acidic ph

The receptors then get concentrated into a recycling compartment of the early endosome

The vesicles of the sorting compartment return the receptors to the cell surface to be used again

Question 20

What is the degradation pathway of endocytosis

Answer 20

The signalling receptor bind to ligands (things) that affect the activities of the cell (hormones, growth factors)

They are first moved to the early endosome for sorting but this time the early endosome matures into the late endosome

The late endosome fuses with the lysosome to degrade the receptors inside it

The degradation of the receptor stops the cell from being stimulated more because less uptake of the hormone/ growth factor is happening, it limits the duration of the signal