Thylakoid Flashcards
what is the targeting sequence for the thylakoid
core hydrophobic domain with positive N term and small side chain aa C term. this is cleaved from stomal transit peptide once in the thylakoid so only the mature protein is found in thylakoid
what are the 2 pathways for import into the thylakoid
the Sec pathway, pH/tat pathway and spontaneous insertion
how was it determined that there are multiple pathways to enter the thylakoid
addition of ionophores which dissipate the pmf inhibited transport of some proteins e.g. OE23 and OE17 (pH pathway) but not others such as Plastocyanin and LHCP (Sec pathway)
describe the homologous bacterial sec pathway
protein is synthesised in the cytosol and bound to chaperones e.g. SecB. secB transfers protein to SecA which is in complex with SecE. Sec A hydrolyses ATP to provide energy for transfer through the pore formed by SecE.
How was it determined that PC uses Sec pathway
recombinant techniques were used to switch targeting signals found that OE23-PC hybrid uses the pH pathway in the same was as wild type OE23 however PC-OE23 is not imported. this is because OE23 is too stable and so cannot be unfolded to go through the sec pathway
what are the differences between the signals of proteins destined for the sec pathway and those destined for the pH pathway
pH pathway proteins have twin arginine residues preceding the hydrophobic core of the transit peptide
describe the bacterial pH/tat pathway mechanism
tatb/c form the receptor for substrate protein. this somehow recruits multiple copies of tatA to form a channel through which folded protein can pass through. this is pmf dependent.
what are the problems posed by transporting folded proteins through the tat pathway and how are these overcome
the pore must be able to accommodate a large variety of shapes of proteins. the recruitment of multiple subunits of tatA is thought to overcome this, however it is calculated that in vitro 30000 protons are lost for each protein translocated through this pathway due to the breakdown of the permeability barrier
what is required for the insertion of thylakoid integral membrane proteins
GTP and stromal factors such as cpSRP which binds internal targeting sequence to form a transit complex in the stroma
what is the evidence for a “spontaneous” insertion pathway in thylakoids
some small integral envelope and thylakoid membrane proteins such as CFoII and OEP14 no requirement for proteins or an energy source can be detected
