cytoskeleton and membrane protein synthesis Flashcards
how may membrane proteins contribute to the cytoskeleton
spectrin is joined end to end to form lattice structure of cytoskeleton
attachment proteins like ankyrin attach it to transmembrane proteins on membrane
what goes wrong if there is an issue with proteins in cytoskeleton of RBC
hereditary spherocytosis- spectrum depleted by 50% so lattice doesn’t form well and RBC round up and are spherical instead of biconcave. they become less resistant to passage through small capillaries and less resistant to lysis and have to be cleared by spleen.
hereditary elliptocytosis- defect in spectrin molecule so end to end association of spectrum molecules(heterotetramers) can’t form. fragile elliptoid cells. again cells are less resistant to sheering stress of going through capillaries so lyse and cleared by spleen
how are secreted proteins (like insulin) synthesised and become membrane proteins?
proteins are made on ribosome as usually as they attach to RNA and read down the RNA (triplet code)
secreted proteins will have a hydrophobic seq of AA on their N terminus.
these are recognised by SRP(signal recognition particle) and bind therefore ribosome can’t elongate the protein.
SRP then is recognised by docking protein and SRP dissociates from ribosome and leaves a free N-terminal signal seq that can be recognised by signal seq receptor in membrane of ER.
As SRP is now not bound it can continue to elongate again.
the signal seq is cleaved by signal peptidase .
when polypeptide chain is completed it is fully extruded in ER lumen
first transmembrane domain of protein acts as a stop transfer seq locking protein into membrane.
the ribosome continues to make the protein so detaches from protein translocation machinery and completes rest of protein in cytoplasm.
why can’t protein enter the membrane translocation machinery head first and how is this overcome
due to the positive charges on n terminal
instead +ve charge bind to the signal seq receptor on cytoplasmic side and instead of diving head first, then signal seq flops in like a hairpin
