Lec10 - speed revieww

Sec61 Complex

The protein translocator channel in the ER membrane that forms a gated aqueous pore for nascent polypeptide translocation.

SRP & SRP Receptor GTPase Cycle

Both bind GTP to form a high-affinity targeting complex at the ER membrane. GTP hydrolysis in both proteins triggers SRP dissociation from its receptor and the ribosome, recycling SRP to the cytosol.

Transamidase

ER luminal enzyme that cleaves the C-terminal hydrophobic tail from a precursor protein and attaches the remaining polypeptide to a preformed GPI anchor.

Double-Pass Protein Insertion

Internal start-transfer sequence initiates translocation, and subsequent stop-transfer halts it. Both exit laterally into the lipid bilayer, leaving the intervening loop in the ER lumen and both N- and C-termini in the cytosol.

Charge Flanking Rule

Flanking charged residues surrounding an internal start-transfer sequence determine orientation: positively charged (++) side stays in cytosol, negatively charged (--) side enters ER lumen.

GPI-Anchored Protein Location

Attached entirely to the luminal leaflet of the ER membrane, causing the protein to face the ER lumen (and ultimately the extracellular cell surface).

Signal Peptidase

ER luminal enzyme that cleaves the N-terminal ER signal sequence during translocation, releasing the protein's N-terminus into the ER lumen.

Single-Pass Type 1 Protein Insertion

Uses a cleaved N-terminal ER signal sequence (start-transfer) and an internal stop-transfer sequence; leaves N-terminus in ER lumen and C-terminus in cytosol.

SRP Binding Requirements

SRP binds with high affinity only when both the ribosome AND the hydrophobic N-terminal ER signal sequence are present, temporarily pausing translation in the cytosol.

Co-Translational vs. Post-Translational ER Import

Eukaryotic ER import is predominantly co-translational (translocation happens simultaneously while the ribosome is actively translating mRNA).