Question

Cell Biology Short Answer Question: In multipass membrane proteins synthesized in association with membrane-bounded ribosomes of the rough ER, signal-anchor and stop-transfer anchor sequences alternate. What do these sequences do? Compare and contrast Type I, Type II and Type III ER membrane proteins.

Answer 1

Signal and stop transfer anchor sequences are hydrophobic sequence that trigger opening of pore sideways, so protein slides out of pore, laterally into lipid bilayer. In the absence of signal peptide, these hydrophobic sequence act as a start transfer sequence that intiate translocation of transmembrane segment whereas in presence of signal peptide, it act as stop transfer sequence that align with the signal peptide in the translocator channel. Multipass protein has multiple signal anchor and stop transfer sequences. Stop transfer sequence stop the passage of polypeptide chain through the translocon to become a hydrophobic transmembrane segment in membrane bilayer, protein to shift out of translocon into the membrane.

Type I. : Single hydrophobic transmembrane stretch

N- terminal on the luminal ( exterior) side of membrane and carboxyl end on the cytoplasmic side of membrane

Machinary : SRP/SR/sec 61/ signal peptidase

Topology : cleavable signal and stop transfer seqence

Type 2 : similiar to type 1 ER protein it is also single pass protein

Carboxy end toward luminal side and amino end toward cytosolic side of membrane.

Topology : signal anchor sequence

Machinary : SRP/SR/ sec61

Stop transfer sequence act as both signal and anchor sequence.

Type 111 : multipass transmembrane protein

Contain reverse signal anchor

Machinary : SRP/ SR/ sec 61

In both type 2 and 3 contain signal anchor sequence internal but serve as both signal sequence and membrane anchor sequence.