Secretory cargo sorting by Ca2+-dependent Cab45 oligomerization at the trans-Golgi network

Abstract

Sorting and export of transmembrane cargoes and lysosomal hydrolases at the trans-Golgi network (TGN) are wellunderstood. However, elucidation of the mechanism by which secretory cargoes are segregated for their release into theextracellular space remains a challenge. We have previously demonstrated that, in a reaction that requires Ca2++, thesoluble TGN-resident protein Cab45 is necessary for the sorting of secretory cargoes at the TGN. Here, we report thatCab45 reversibly assembles into oligomers in the presence of Ca2++. These Cab45 oligomers specifically bind secretoryproteins, such as COMP and LyzC, in a Ca2++-dependent manner in vitro. In intact cells, mutation of the Ca2++-binding sitesin Cab45 impairs oligomerization, as well as COMP and LyzC sorting. Superresolution microscopy revealed that Cab45colocalizes with secretory proteins and the TGN Ca2++ pump (SPCA1) in specific TGN microdomains. These findingsreveal that Ca2++-dependent changes in Cab45 mediate sorting of specific cargo molecules at the TGN.