The sorting nexin requirement of the Vps68 recycling signal is context dependent

Here we analyzed the recycling of the Vps55/Vps68 complex in the yeast endocytic pathway. Deletion of VPS55 and VPS68 caused the same moderate stabilization of the endocytic cargo protein Ste6. No additive effect was observed by the double deletion, reinforcing the notion that both proteins form a functional unit. This is further underlined by the finding that the two proteins are dependent on each other for proper cellular localization. A tyrosine-based recycling signal was identified in the cytosolic tail of Vps68. Curiously, it turned out that the recycling signal was context dependent with respect to the usage of recycling factors. In its natural context, recycling was dependent on the sorting nexin Mvp1/SNX8 and independent of retromer. But, when the signal was inserted into a well-studied retromer substrate, the CPY receptor Vps10 devoid of its own signals, it became dependent on retromer and Snx3. This finding suggests that the availability of the recycling signal could be subjected to regulation. Previously, we obtained evidence that Vps68 cooperates with ESCRT-III in intraluminal vesicle (ILV) formation at late endosomes. It is thus conceivable that recycling of Vps55/Vps68 only occurs when its function in ILV formation is finished. Our data also suggest that recycling of Vps55/Vps68 could be regulated by phosphorylation. In addition, we identified Dcr2, the yeast orthologue of human sphingomyelin phosphodiesterase acid-like 3A (SMPDL3A), as a new factor involved in Mvp1 dependent recycling of Vps55/Vps68. Article SummarySome proteins in the endocytic pathway are recycled to the Golgi or to the plasma membrane and are thus spared from degradation in the lysosome or vacuole. Here a recycling signal was identified in the Vps55/Vps68 complex. The data suggest that the accessibility of the signal is regulated. Further, a new factor involved in the sorting nexin Mvp1/SNX8 dependent recycling of Vps55/Vps68 was identified. This factor is Dcr2, the yeast orthologue of human sphingo-myelin phosphodiesterase acid-like 3A (SMPDL3A).