Final answer:
The key structural protein in the cortex of red blood cells is spectrin, which provides their shape and flexibility, allowing them to move through the circulatory system.
Step-by-step explanation:
The principal fibrous protein in the cortex of the red blood cell is spectrin.
In the structural hierarchy of protein functions, we find a variety of proteins with different roles. One critical role is that of the organizational framework within cells, known as the cytoskeleton, which is integral for maintaining cell shape and structural integrity.
The principal component of the cortex beneath the plasma membrane of red blood cells is the cytoskeleton, which is largely made up of an interconnected network of fibrous proteins. Spectrin is the key fibrous protein of the cytoskeleton in red blood cells. It is not soluble in water and performs a critical structural function, giving red blood cells their characteristic biconcave shape and flexibility, allowing them to traverse the narrow capillaries of the circulatory system.
This actin cortex is also instrumental in the dynamic aspect of the cell's response to mechanical stresses, with actin filaments and other linker molecules contributing to this structure alongside spectrin. While other proteins like keratin and myosin also have structural roles within their respective domains—keratin in hair, nails, and skin and myosin in muscle contraction—within the context of the red blood cell, it is spectrin that serves as the principal fibrous component.