Which types of proteins are directed to the ER?

Proteins directed to the endoplasmic reticulum (ER) include soluble proteins and transmembrane proteins. This is because the ER is the site where these proteins undergo synthesis and initial folding. Soluble proteins, such as secreted proteins and those that reside in the ER lumen, are translocated into the ER as they are synthesized. These proteins will ultimately be processed and may be sent to various destinations, including the extracellular space or different organelles.

Transmembrane proteins are also directed to the ER, where they are inserted into the lipid bilayer during their synthesis. These proteins are integral parts of the cellular membrane system, including those that reside in the plasma membrane or are associated with various organelles.

The other options include proteins that typically do not interact with the ER. Enzymatic and structural proteins can be formed in the cytoplasm and may not require delivery to the ER. Cytosolic proteins, by definition, do not go to the ER, and nuclear proteins and cytoskeletal proteins are usually synthesized in the cytoplasm and have different destinations within the cell. Therefore, soluble and transmembrane proteins are the correct types that are directed to the ER for processing and maturation.