Final answer:
Integrin is the protein that attaches a crawling cell to the substrate in a focal adhesion.
Step-by-step explanation:
The protein that attaches a crawling cell to the substrate in a focal adhesion is integrin. Focal adhesions are complex structures that connect cells to the extracellular matrix. Integrins are transmembrane adhesion receptors that bind to the extracellular matrix on the outside and are crosslinked by cytoplasmic proteins such as talin on the inside. These connections allow the cell to adhere to the substrate and provide a link between the actin cytoskeleton and the extracellular environment.