ADF/cofilin regulates secretory cargo sorting at the TGN via the Ca2+ ATPase SPCA1

Actin-severing proteins ADF/cofilin are required for the sorting of secretory cargo at the trans-Golgi network (TGN) in mammalian cells. How do these cytoplasmic proteins interact with the cargoes in the lumen of the TGN? Put simply, how are these two sets of proteins connected across the TGN membrane? Mass spectrometry of cofilin1 immunoprecipitated from HeLa cells revealed the presence of actin and the Ca(2+) ATPase SPCA1. Moreover, cofilin1 was localized to the TGN and bound to SPCA1 via dynamic actin. SPCA1 knockdown, like ADF/cofilin1 knockdown, inhibited Ca(2+) uptake into the TGN and caused missorting of secretory cargo. These defects were rescued by the overexpression of the TGN-localized SPCA1. We propose that ADF/cofilin-dependent severing of actin filaments exposes and promotes the activation of SPCA1, which pumps Ca(2+) into the lumen of the TGN for the sorting of the class of secretory cargo that binds Ca(2+).