Explanation:

- Collagen: A structural protein found in skin and connective tissue, but not specifically protective of epithelial cells.

- Elastin: Gives elasticity to tissues, allowing them to resume shape, but isn't specifically protective of epithelial cells.

- Actin: A protein contributing to cellular movement and structure but doesn't provide protective functions to epithelial cells.

- Keratin: This durable protein protects epithelial cells from damage and stress. It is abundant in skin, hair, and nails, acting as a barrier.