intracellular protein trafficking

Question 1

intracellular protein trafficking

Answer 1

each organelle is characterised by a specific set of proteins ( because each organelle has a specific function and this function is carried out by protein). the general rule is that each protein is only found at one location. most organelles have luminal (dissolved inside) and membrane proteins. the only exception is the Golgi apparatus, where the soluble proteins are in transit to other destinations.

Question 2

signals for directing proteins destination

Answer 2

signals are required to direct proteins from their site of synthesis in the cytosol to their different destinations- where they perform their functions
just like a zip code system - same zip code means same destination.

Question 3

types of sequence

Answer 3

no signal sequence - cytoplasm
nuclear localisation sequence - mitochondria
ER signal sequence - plasma membrane , resident ER and Golgi , endosomal and lysosomes

Question 4

the Golgi apparatus

Answer 4

the Golgi apparatus is the sorting organelle of the cell .
proteins on the rough ER are sent to the Golgi as they move through the Golgi apparatus they are modified.

Question 5

the secretory pathways

Answer 5

constitutive and regulated

Question 6

example of proteolytic processing taking place In the Golgi

Answer 6

processing of insulin ( synthesised in the ER of the pancreas-b cells

cleavage of signal peptide by signal peptidase .
disulphide bond formation .
further trimming by a caboxypeptidase B like enzyme removes the 2 basic residues from each of the new ends.
cleavage bt trypsin like enzymes release the C-peptidase.
the C chain is packaged in the secretory vessicle and is secreted along with active insulin.

Question 7

translocation step

Answer 7

proteins that are translocated into the lumen have a signal sequence - this has a major characteristic : 7-12 hydrophobic aas row. in contrast transmembrane proteins have transmembrane segments that are composed of about 20 hydrophobic amino acids.

Question 8

signalling from translocation of a secretory protein

Answer 8

a major advance in this field came with the postulate of the signal hypothesis. you’re starting out with the translation of a secretory protein - the ribosome starts from the 5’ end of MRNA and then moves towards the 3’ end synthesising the N-termius first. the signal sequence directs the ribosome to the membrane , the ribosome binds to a channel peptide chain and is moved through the membrane as it is being made by the ribosome. eventually the signal sequence is cleaved off- a new N-termius is generated now in the lumen. the protein is then completed and ends up in the lumen of the endoplasmic reticulum . the ribosome will eventually disassociate into its 2 subunits and then you can start a new cycle.

Question 9

examples of secretory proteins

Answer 9

polypeptide hormones (insulin)
albumin
collagen
immunoglobulins

Question 10

integral membrane proteins are also synthesised by the same mechanisms . examples of integral membrane proteins :

Answer 10

polypeptide hormone receptors
transport proteins
ion channels
cytoskeletal anchoring proteins

Question 11

newly synthesised polypeptides in the membrane and lumen of the ER undergo 5 principal modifications

Answer 11

formation of disulphide bonds
proper folding
addition and processing of carbs
specific proteolytic cleavages
assembly into multimeric proteins