 The first step in the COP II pathway is the recruitment of a small GTPase named Sar1 to the ER membrane.Once Sar1 interacts with the ER membrane, a membrane protein called Sec12 acts a guanine nucleotide exchange factor and substitutes GTP for GDP on Sar1.This activates the Sar1 protein, causing its amphipathic alpha helix to bind to the ER membrane.