A PKD-caveolin axis drives secretory carrier biogenesis at the TGN

Constitutive secretion from the trans-Golgi network (TGN) to the cell surface proceeds via carriers thought to form without a canonical cytoplasmic coat, yet how these carriers are generated remains poorly understood. Here, we identify a distinct population of TGN-to-cell surface carriers transporting influenza hemagglutinin (HA) and uncover a coat-like mechanism underlying their formation. HA carrier biogenesis requires non-vesicular lipid transfer at endoplasmic reticulum (ER)-Golgi membrane contact sites (MCSs) and protein kinase D (PKD) activity. We show that caveolin promotes membrane budding by assembling into cholesterol- and PKD-associated oligomers that act as a membrane-embedded, coat-like scaffold at lipid nanodomain-enriched TGN subdomains. These findings establish caveolin as a structural and regulatory component of TGN export and support a model in which a PKD-caveolin axis couples ER-Golgi lipid transfer to cargo sorting, membrane remodeling and fission during secretory carrier biogenesis.