Sar1 is a small GTPase protein that plays a crucial role in the transport of proteins within cells. It is part of the Sar1/Arf family, which is involved in the formation of vesicles that carry proteins from the endoplasmic reticulum to the Golgi apparatus. When activated, Sar1 binds to the membrane of the endoplasmic reticulum, initiating the budding process of transport vesicles. This process is essential for maintaining proper cellular function and ensuring that proteins reach their intended destinations within the cell.