Sar1 GTPase is a small GTP-binding protein that initiates COPII coat assembly at the endoplasmic reticulum. Its GTP-bound state associates with the membrane and recruits the Sec23-Sec24 complex, followed by the Sec13-Sec31 complex. Nucleotide cycling helps coordinate coat assembly and turnover. Reconstitution with defined components demonstrates a direct role in carrier formation rather than only an indirect consequence of cellular signalling. [ref]
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