The Sec23-Sec24 complex forms the inner COPII coat layer. Sec24 provides cargo-recognition sites, while Sec23 binds Sar1 GTPase and contributes to regulation of its GTPase cycle. The outer Sec13-Sec31 complex then organizes the coat scaffold. Distinct cargo-binding sites on Sec24 explain how one coat can capture different cargo classes rather than relying on a single universal recognition sequence. [ref]
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