The yeast DENN domain protein Avl9 contributes to recycling and sorting of endosomal cargos

In yeast and humans, the conserved DENN ('differentially expressed in normal and neoplastic tissue') domain protein Avl9 is thought to play roles in membrane traffic and secretion, but its precise function remains poorly defined. Since DENN domain-containing proteins are associated with Rab GTPase function, we sought to understand Avl9 function in the context of Rab regulation. Here, we show that Avl9 localizes to peripheral punctae that are consistent with secretory vesicles. Moreover, we demonstrate genetic interactions and colocalization between Avl9 and numerous Rab family proteins in the secretory and endosomal pathways, suggesting a potential function at the interface of secretion and recycling. Consistent with this role, avl9Δ results in defective recycling of the endosomal cargo Snc1 but does not alter plasma membrane delivery of an endocytosis-defective Snc1EN- mutant, suggesting that Avl9 is not solely involved in secretory traffic from the trans-Golgi network to the plasma membrane. The avl9Δ recycling defect is exacerbated by additional loss of RCY1 or SNX4, but not VPS35. Each of these three genes contributes to a distinct endosomal recycling pathway, indicating that Avl9 acts in conjunction with multiple recycling pathways.