What fibrous protein associates with the cytoplasmic portions of cell-surface receptors that bind lipoproteins?

Clathrin is the fibrous protein that associates with the cytoplasmic portions of cell-surface receptors that bind lipoproteins. It plays a crucial role in endocytosis, where it helps to form coated pits on the cytoplasmic side of the plasma membrane. When a receptor binds a lipoprotein, clathrin helps to facilitate the invagination of the membrane, leading to the internalization of the receptor-lipoprotein complex into the cell.

Clathrin forms a characteristic basket-like structure that promotes the budding off of vesicles from the membrane. This process is essential for the uptake of various molecules, including lipoproteins, which are important for cellular functions such as metabolism and signaling.

In contrast, actin is primarily involved in cellular shape and movement; collagen provides structural support in connective tissues; and keratin is a key structural protein found in hair, nails, and the outer layer of skin. These proteins do not directly associate with the receptors for lipoproteins in the same functional context as clathrin. Hence, clathrin is specifically linked to the endocytic pathway involving the binding of lipoproteins to their receptors, making it the correct answer to the question.