Protein Synthesis and targeting Flashcards
How proteins get where, primary sequences
Where are proteins synthesized?
On ribosomes in the cytoplasm
Explain co-translational sorting
A short stretch of 6-12 NON polar amino acids at the N terminal (signal sequence) targets protein to the endomembrane system.
Proteins are packed in transit vesicles where they are transported to the golgi with the help of motor proteins.
Where can proteins go after being packaged into vesicles in co-translational sorting?
-constitutive release - vesicles are transported to the plasma membrane where they immediately release contents
-Regulated secretion - vesicles move to the cell surface where they wait for a signal
-others become lysosomes
Plasma membrane proteins
TM domain of 18-22 NON POLAR animo acids, recognized by the SRP receptor and released into membrane
Endoplasmic Reticulum resident proteins
Carry a retrieval sequence, usually the last 4 amino acids at the C terminal, this allows proteins to be retrieved from golgi when they accidentally enter transit vesicles leaving the ER. They also cary a signal at the N terminal.
Lysosomal proteins
A signal patch on lysosomal proteins allows them to be targeted with manose-6-phosphate in the golgi and sorted to immature lysosomes. they aso carry a signal sequence at the N terminal/
Post-translational sorting to the mitochondria
molecular chaperones come along and prevent the protein from folding into its 3 configuration
Mitochondrial proteins enter the matrix through a complex called TOM. TOM then reorients the protein, inserting the presequence into the translocon.
PTS to Mitochondria: Destined for IMS
these proteins will simply move through TOM and its sequence will be clipped by an enzyme, and other enzymes help it fold into its 3 configuration.
PTS to Mitochondria: Destined for Matrix
Protein makes its way through TOM and into the IMS. Then proteins on TIM 23 bind the signal sequence, while PAM and energy from ATP pull the protein through TIM into the matrix. It is then clipped and folded by enzymes.
PTS to Mitochondria: Destined for IMM
these proteins have 2 signal sequences. Presequence brings it through TOM. Protein is becomes anchored to membrane, TIM 22 opens up and allows protein to move laterally into IMM.
Post translational sorting to the Nucleus
Nuclear proteins are synthesized in the cytoplasm, and fall off their ribosomes. These proteins contain a nuclear localization sequence (NLS) found anywhere on the protein. Importin a and b bind to the protein and move it into the nucleus. Protein Ran helps remove the importins. Ran binds to importin b and moves out to cytoplasm to be recycled. a importin subunit needs help, so it binds to GTP and exportin and leave nucleus. OUT THERE, Ran GAP binds to Ran+GTP, releases a phosphate group. Ran is now bound to GDP. NTF 2 binds to Ran GDP, bringing it BACK into the nucleus. Ran GEF binds to Ran GDP, releasing NTF2 and GDP, and picks up a GTP
