How are soluble proteins sorted to the ER?

Soluble proteins are sorted to the endoplasmic reticulum (ER) through a specific mechanism involving an ER signal sequence. This sequence, often located at the N-terminus of the nascent protein chain, is recognized and bound by a receptor in the ER membrane. This interaction is crucial because it facilitates the translocation of the protein into the ER lumen, where it can undergo further folding and modifications.

The presence of the ER signal sequence is essential in this sorting process, as it serves as a molecular tag that distinguishes proteins destined for the ER from those that will follow different pathways. Once the receptor binds to the signal sequence, it often initiates the process of translation and translocation, allowing the protein to enter the ER while it is still being synthesized.

In contrast, the other options do not accurately describe this sorting mechanism. For instance, directing proteins to the nucleus involves different signal sequences and transport pathways, and structural properties alone do not determine the destination of soluble proteins in cells. Additionally, coiling around the lipid bilayer does not occur in the context of soluble protein sorting to the ER, as these proteins are typically translocated into the ER lumen rather than integrating into the membrane.