What kind of proteins does the smooth ER primarily deal with?

The smooth endoplasmic reticulum (ER) primarily deals with lipid-synthesizing proteins due to its key role in lipid metabolism. The smooth ER is involved in the synthesis of various lipids, including phospholipids and cholesterol, which are essential components of cell membranes. It also plays a crucial role in the production of steroid hormones and triglycerides.

In addition to lipid synthesis, the smooth ER is involved in detoxification processes and carbohydrate metabolism. The abundance of smooth ER is particularly notable in cells that are specialized for these functions, such as liver cells, which require extensive lipid metabolism for detoxification.

While the other types of proteins listed, such as signal proteins, structural proteins, and secretory proteins, are important within the cell, they are primarily associated with other cellular structures like the rough ER, which is studded with ribosomes for protein synthesis, particularly for those destined for secretion or for use in membranes. Hence, the smooth ER's primary function aligns specifically with lipid-synthesizing proteins.