Post-Translational Events I

Translocation of Newly Synthesized Proteins into Membranous Organelles

Locations of Protein Synthesis

• ER Associated Ribosomes– proteins translocate co-translationally into ER

lumen– subsequent processing & transport through

Golgi for secretory vesicles and lysosomes

• Cytoplasmic Ribosomes– proteins translocated post-translationally into

membranous organelles (except Golgi, ER and lysosome)

Transport of Cytoplasmically Synthesized Proteins

• Import into organelles (transmembrane and lumenal)– Mitochondrial import– Chloroplast import– Peroxisome import

• Translocation across plasma membrane

• Insertion in cytoplasmic face of plasma membrane

Signal Sequences

• Portions of protein sequence– Frequently N-terminal, C-terminal– Occasionally located within– Often cleaved following transport event

• Signal Recognition Particles/Complexes– protein-translocation channels– escort (chaperone) proteins

Mitochondrial Protein Import

• Four Destinations– Outer membrane– Innermembranous space– Inner membrane– Matrix

• TOMs– Translocase of the outer

mitochondrial membrane

• TIMs– Translocase of the inner

mitochondrial membrane

• Tiny TIMs– escorts of the innermembranous

space

TOM & TIM Function

Tiny TIMs and TIM22

TOC & TIC

Peroxisome Import

Signal sequence for recognition by PEX5

-SKL-COOH

Peroxisome Formation

Translocation of ER-Associated Ribosome Synthesized Proteins

Docking of Ribosomes to ER

Signal peptides typically 10-20 residues – shorter than those on cytostolically synthesized proteins

Signal Recognition Particle• Ribonucleoprotein complex• Similar to bacterial SRP

Ribosome binding Signal peptide binding

translocation

Translocation of Various Classes of ER Proteins

lumenal proteins

Type 1 transmembrane proteins

Type 2 transmembrane proteins

multipass transmembrane proteins