Practice question

ER export mediated by COPII coats discovered through sec mutants in yeast by Schekman. At ER exit sites devoid of ribosomes, integral GEF Sec12 converts Sar1 GDP to GTP causing amphipathic helix insertion into ER membrane inducing curvature and recruiting inner coat Sec23-Sec24 heterodimer. Sec24 isoforms provide multiple cargo binding sites recognizing di-acidic DXE, di-hydrophobic, proline-rich and arginine motifs concentrating transmembrane and soluble cargo via receptors Erv29, Erv14, Surf4 and Erv29. Outer coat Sec13-Sec31 heterotetramer forms cage driving deformation and budding producing 60-80 nm vesicles that uncoat after Sar1 hydrolysis mediated by Sec23 GAP activity. Vesicles fuse forming ERGIC then cis-Golgi mediated by SNAREs Sed5, Bet1. COPI drives opposite Golgi-to-ER retrieval, clathrin operates at TGN and plasma membrane for lysosomal sorting and endocytosis. Temperature-sensitive Sar1 mutants block secretion trapping cargo in ER demonstrating COPII essential for anterograde ER to Golgi leg conserved from yeast to human and plants needing precise export code.