When Translating Secretory or Membrane Proteins,ribosomes are Directed to the ER

Question

When translating secretory or membrane proteins,ribosomes are directed to the ER membrane by
A)a specific characteristic of the ribosome itself, which distinguishes free ribosomes from bound ribosomes.
B)a signal-recognition particle that brings ribosomes to a receptor protein in the ER membrane.
C)moving through a specialized channel of the nucleus.
D)a chemical signal given off by the ER.
E)a signal sequence of RNA that precedes the start codon of the message.

Accepted Answer

Secretory or membrane proteins are directed to the ER membrane by a signal-recognition particle (SRP) that recognizes a signal sequence of amino acids on the nascent polypeptide and brings the ribosome to a receptor protein in the ER membrane. This allows for co-translational translocation of the protein into the ER lumen or membrane.