KEY MECHANISMS OF PROTEIN TRANSPORT
- Gated transport: energy-dependent (nuclear pores)
- Translocation across membranes: mediated by protein translocators (mitochondrion, post-translational and cotranslational import into ER or peroxisomes)
- Vesicular transport: proteins do not cross membranes (from ER to Golgi, through Golgi stacks or to plasma membrane)
PROTEIN TRANSLATION
• Synthesis of almost all proteins begins in the cytoplasm
• 3 possible outcomes depending on signal sequence (or lack thereof):
i. remain in cytosol ii. post-translational import into organelle (mitochondrion, peroxisome, nucleus, ER) iii. cotranslational import to ER
SIGNAL SEQUENCES
• Short amino acid sequences at the terminal end of proteins (address labels)
• 15-60 residues long
• Direct proteins to specific organelles
• Proteins without signal sequences remain in the cytosol
MEMBRANE TOPOLOGY
• Compartments are topologically equivalent if molecules can get from one to another without having to cross a membrane
• ER, Golgi, vesicle, perinuclear space in nuclear envelope are all topologically equivalent
• Mitochondrion, nucleus and peroxisomes are topologically distinct
