Which component is responsible for the physical formation of the vesicle coat?

The physical formation of the vesicle coat is primarily facilitated by clathrin. Clathrin is a protein that plays a crucial role in the process of endocytosis and vesicle trafficking within cells. During the formation of clathrin-coated vesicles, clathrin molecules assemble into a basket-like structure on the cytoplasmic surface of membranes, which helps to invaginate the membrane and form a bud. This process is essential for creating transport vesicles that carry proteins and other molecules to various destinations within the cell.

Other components mentioned, like monomeric GTPases and Rab GTPases, are involved in signaling and regulation of vesicle transport but do not directly form the coat of the vesicle themselves. SNAREs, on the other hand, are proteins that mediate the fusion of vesicles with their target membranes rather than participating in the vesicle's initial formation. Thus, clathrin is indeed the key protein responsible for the physical structure of the vesicle coat.