A protein that plays a role in both coagulation and platelet aggregation is:

Factor I, also known as fibrinogen, is a crucial protein in the coagulation cascade that directly contributes to both coagulation and platelet aggregation. When a blood vessel is injured, the coagulation process is initiated to prevent excessive bleeding. Fibrinogen is converted into fibrin by the action of thrombin, which helps to form a stable clot. Additionally, fibrinogen acts as a bridge between platelets, promoting their aggregation. This dual role makes fibrinogen essential not only for the formation of a blood clot but also for the effective aggregation of platelets at the site of injury.

While factors like VIII, IX, and XI are integral to the coagulation cascade, they primarily function as procoagulants that activate other factors rather than directly promoting platelet aggregation. Thus, they do not play the same dual role in both coagulation and platelet aggregation as fibrinogen does.