How do proteins become embedded in the membrane of the endoplasmic reticulum?a. by splicing of proteins on either side of the ER membrane. b. via stop- and start-transfer sequences that stop and start translocation of a polypeptide through the ER membrane, resulting in transmembrane proteins. c. by post-translational import can results in a transmembrane ER protein. d. by protein chaperones that insert polypeptides into the ER membrane. e. via transit sequences that target polypeptides made in the cytosol for the ER membrane.

The correct answer is b.

Explanation:

The signal peptide acts as an initiator of the  transfer. Proteins whose first destination is the endoplasmic reticulum have a signal peptide at their amino terminal end that includes a sequence composed of 5 to 10 hydrophobic amino acids, which is recognized by a receptor of this organelle that opens a channel in its membrane allowing such proteins to can get through it. Most of these proteins will then continue their course towards the Golgi apparatus, but those that also have a certain sequence of four amino acids (-Lys-Asp-Glu-Leu-COOH) at their other end (carboxyl terminal) are retained in the endoplasmic reticulum as proteins of this organelle. The transfer stop peptide anchors the protein on the membrane after the ER signal peptide (transfer initiator) is released from the translocator and removed.