Intracellular processes

Question 1

How do proteins enter organelle/ go where they need to go?

Answer 1

Nuclear pores, protein translocators, transport vesicles from the ER for extracellular secretion, plasma membrane or forming a lysosome

Question 2

How do ribosomes know to go to ER?

Answer 2

signal peptide on proteins being made guided to ER by Signal-recognition particle (SRP) and SRP receptor
1. SRP in cytosol binds to ER signal peptide when exposed on ribosome
2. SRP receptor embedded on ER membrane binds to SRP (which is now attached to signal peptide and ribsosome)
3. Protein continues to be produced and is thread through translocon in ER membrane
4. SRP disociates and is reused for a different protein
5. Protein is encapsulated in transport vesicle and secreted from ER

Question 3

What is a signal protein?

Answer 3

specific sequence of N-terminal amino acids (start of protein)

Question 4

Describe the ‘post office’ function of the golgi apparatus

Answer 4

Recieves proteins from one location and targets them to an other using wave like movements (Cis Cisterna becomes pat of Medial Golgi Cisterae, new Cis C is formed behind, etc.)
1. Vesicles of translated proteins from ER fuse with each other to form the Cis Cisterna
2. Proteins move through stack and modified by resident golgi enzymes - providing signal for final destination of protein
3. Proteins sorted in Trans C - proteins with same target sequence placed in same place
4. Trans C buds off into vesicles which move towards target location

Question 5

Vesicle pathways after golgi

Answer 5

1. Exocytosis: proteins remain on ER membrane and vesicle ultimately fuses with plasma membrane
2. Endocytosis: specific sugar chain added to protein labelling it for final destination (e.g. mannose-6-phosphate added to proteins destined for lysosomes)

Question 6

Gylocosylation

Answer 6

Specific sugar added to ‘core oligosaccharide’ attached to protein acting as signal for final destination. E.g. addition of mannose-6-phosphate for lysosomes

Question 7

Proteglycans

Answer 7

Extensively glycosylated proteins

Question 8

Glycoproteins

Answer 8

Proteins with small sugar component

Question 9

Phosphorylation

Answer 9

addition of a phosphate group, alters activity of proteins, hyperphosphorylation of Tau protein is hallmark of neurodegenative disease

Question 10

Acetylation

Answer 10

Addition of an acetyl group, regulation of gene expression in histones

Question 11

Ubiquitination

Answer 11

Addition ubiquitin chain, targets protein for degradation

Question 12

Farnesylation

Answer 12

Addition of farnesyl group, targets proteins to cytoplasmic face of plasma membrane

Question 13

Lysosomal degradation

Answer 13

In lysosomes, lysosomal enzymes activated by acidic environment of lysosome
Used for: proteins with long half life (autophagy), membrane protein brought in by endocytosis, extracellular proteins (brought in by receptor mediator endocytosis), phagocytic proteins (brought in via phagocytosis)

Question 14

Proteasomal degradation

Answer 14

In cytosol at proteasomes (cylidrical protein complexes), walls made from protease enzymes with active sites inside, protein stopper at each end, ATP dependent.
Used for: proteins with short half life, needs to be removed quickly, key metabolic enzymes and defective proteins

Question 15

Mechanism of Proteasomal degradation

Answer 15

1. Proteins tagged with Ubiquitin
2. Shuttling proteins take ubiquitinated proteins into proteasome
3. Tagged proteins are recognised, unfolded and translocated
4. Degraded into peptides
5. Peptides extruded and digested by cytosolic peptidases