The alternate AP-1 adaptor subunit Apm2 interacts with the Mil1 regulatory protein and confers differential cargo sorting

Abstract

Adaptor complexes are important for cargo sorting in clathrin-coated vesicles. The µ adaptor subunits Apm1 and Apm2 create functionally distinct versions of the yeast AP-1 complex. A novel regulatory protein is identified that selectively binds Apm2-containing complexes and contributes to their membrane recruitment.

Adaptor protein complex-1 (AP-1) is recruited by the HEATR5 protein Laa1 and its co-factor Laa2 in yeast

“…To monitor AP-1C, we used calcofluor white (CFW) sensitivity in cells lacking the exomer subunit Chs6. This assay is thought to monitor the AP-1c-dependent retention of the chitin synthase Chs3 in the TGN when its normal traffic out of the TGN is disrupted (29,32). We found that laa2⌬ and laa1⌬ caused similar increases in the CFW sensitivity in cells lacking Chs6, suggesting a similar defect in AP-1C-mediated traffic (Fig.…”

“…The top correlated genes also included deletion of MIL1, a recently identified co-factor for AP-1, LAA1, and TRS33, a core component of the TRAPP tethering complexes that function at the Golgi and endosomes ( Fig. 1A) (29,30). The top hits also included one uncharacterized gene YBL010c; on the basis of the results described below, we refer to YBL010c as LAA2 (Large Adaptin Accessory 2).…”

“…Next, we investigated effects of laa2⌬ and laa1⌬ on AP-1 functions. In yeast, AP-1 forms two functionally distinct complexes defined by their incorporation of one of two alternate -subunits (29). The more well-studied AP-1 complex in yeast (referred to here as AP-1C for conventional) contains the -subunit encoded by Apm1, whereas the newly characterized AP-1 complex (AP-1R) contains the -subunit encoded by…”

“…To monitor AP-1R, we used localization of Sna2 Y75A and sertraline sensitivity. Sna2 Y75A is a vacuolar membrane protein whose delivery to the vacuole depends on AP-1R (29). Sertraline is a cationic amphiphilic drug that is thought to interfere with membrane trafficking pathways by intercalating into phospholipid bilayers (33).…”

The AP-1 complex is essential for fungal growth via its role in secretory vesicle polar sorting, endosomal recycling and cytoskeleton organization

“…Yeast AP-1 null mutants also have minor defects in lipid PtdIns(3,5)P2-dependent processes and show reduced ability to traffic ubiquitylated cargoes to the vacuole lumen (Phelan et al, 2006). Notably, in S. cerevisiae , there are two forms of AP-1 which share the same large (Apl2 and Apl4) and small (Aps1) subunits, but distinct medium subunits (Apm1 or Apm2) that seem to confer differential cargo recognition and sorting (Valdivia et al, 2002; Renard et al, 2010; Whitfield et al, 2016). Additionally, in yeast, the AP-1 complex seems to co-operate with the exomer, a non-essential, fungal-specific heterotetrameric complex assembled at the trans-Golgi network, for the delivery of a distinct set of proteins to the plasma membrane (Hoya et al, 2017; Anton et al, 2018).…”