Protein transport 3

Question 1

Overview

Answer 1

• Sec61a,b,y (mammals) = SecY,G,E (e coli)

Question 2

General structure of seca/secy

Answer 2

• Pseudo 2D symmetry
• Channel = large a subunit that spans membrane
• a subunit = N/C 1/2, surrounded by pore, linked by loop btw TM5/6
• y subunit has amphipathic helix + TM segment
• Central aq cavity
• Lateral gate bordered by H2 + 7

Question 3

Truncated mRNA

Answer 3

• Ribosome stalling, removes stop codon → cell stuck in same point of translation
• In vitro w/ 1st 86 codons of preprolactin, imaged structure
• Signal peptide in opposite orientation
• Exit tunnel leads to sec61 translocon
• Ribosome binding to Sec61 is mediated primarily through electrostatic int. btw exit tunnel/ L8 + C terminal residues

Question 4

Channel opening + sequence signal recognition

Answer 4

• Plug moves from position at front of channel to back, opens pore
• ↑ interactions compared to only 2 + 7
• Signal sequence = tilted 45o relative to plane of membrane
• Sequence inserts into primed channel, charged N terminus encounters -ve head of phospholipid
• Charge interactions retain N terminus on cytosolic side of membrane

Question 5

Membrane protein interactions

Answer 5

• Both TM + signal sequences dock onto same site outside lateral gate
• Example = secretory protein
• Membrane proteins inserted by lateral exit of their TM segment in lipid phase
• Multispanning protein integrated by alternating opening + closure of channel

Question 6

How do ions x leak through

Answer 6

• Diameter = 13A
  1. Narrow part + tight packing excludes
  2. Residues on either side of pore ring = hydrophobic, if protein presses tightly around peptide sets up interactions
  3. Hydrophobic gate, out of solution = hydration shell, w/ hydration shell