Question

All transmembrane proteins:

a.contain one or more stop-transfer sequences.

b.None of these are correct.

c.are brought to the ER by chaperones in an unfolded state.

d.always have the N-terminal on the luminal side and the C-terminal on the cytoplasmic side.

e.contain an N-terminal signal sequence.

Answer 1

a) All transmembrane proteins contain one or more stop-transfer sequence. Stop transfer sequence is about 8-14 hydrophobic amino acid residue. It follows either an N-terminal sequence or a start transfer sequence. It is membrane crossing domain and it remains in the membrane. encountering with translocation channel cause deconstruction of the membrane.

b) this option is not correct.

c) No, all proteins are not brought to ER by chaperones in the unfolded state only a few proteins of yeast brought in the unfolded state.

d) No, it is not a universal law, Most of the single pass transmembrane protein posses N-terminal segments on the exoplasmic face and hydrophilic C-terminal segments on the cytosolic side but some transmembrane protein like transferrin receptor and asialoglycoprotein posses the N-terminus on the cytosolic face and C-terminus on exoplasmic (luminal) face.

e)All the protein synthesis is done on the free ribosome that is present in the cytosol. Ribosome engaged with protein synthesis are destined to enter into ER by a signal sequence which is presents on the N-terminal of growing polypeptide chain. The N-terminal signal sequences are a short stretch of 6-12 hydrophobic amino acid that is usually cleaved from polypeptide chain into ER by the N-signal peptidase.