Which protein is known for contributing to the toughness of the skin?

The protein that is primarily known for contributing to the toughness of the skin is collagen. Collagen is a structural protein found in the extracellular matrix of tissues, providing strength and support. It plays a critical role in maintaining the skin's firmness and elasticity, contributing to its overall resilience and ability to withstand stress.

Elastin, while also important for skin elasticity, primarily allows the skin to stretch and return to its original shape rather than providing toughness. Keratin is a key component of the skin's outer layer and is essential for waterproofing and protection but does not directly contribute to the underlying strength that collagen provides. Fibrin is involved in blood clotting and tissue repair but does not have a significant role in the skin's structural integrity. Therefore, collagen is the main protein responsible for the toughness of the skin.