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

Ribosomes are directed to the ER membrane by a signal recognition particle (SRP) that recognizes a signal sequence in the emerging polypeptide and brings the ribosome to a receptor protein in the ER membrane. This allows for the proper localization and insertion of secretory or membrane proteins into the ER membrane.