Vrl1 relies on its VPS9-domain to play a role in autophagy in Saccharomyces cerevisiae

Autophagy is an intracellular degradation process involving many Atg proteins, which are recruited hierarchically to regulate this process. Rab/Ypt GTPases and their activators, guanine nucleotide exchange factors (GEFs), which are critical for regulating vesicle trafficking, are also involved in autophagy. Previously, we reported that yeast Vps21 and its GEF Vps9 are required for autophagy. Later, a third yeast VPS9-domain-containing protein, VARP-like 1 (Vrl1), which was identified as a mutant in major laboratory strains, had partially overlapping functions with Vps9 in trafficking. In this study, we showed that Vrl1 performed roles in autophagy, and its VPS9-domain was crucial for its role in autophagy. We found that localization of Vrl1 differed from the other two VPS9-domain-containing proteins, Vps9 and Muk1, and only Vrl1 changed from multipoint to diffusion after starvation. Like Vps9, Vrl1 suppressed autophagic defects caused by the VPS9 deletion. We further showed that these VPS9-domain-containing proteins, Vps9, Muk1, and Vrl1, all co-localized with Atg8 on autophagosomes in cells blocked in any late step of starvation-induced autophagy, with Vrl1 most often co-localizing with Atg8. A small portion (<25%) of these VPS9-domain-containing proteins were degraded through autophagy. However, a large portion (>60%) of Vrl1 decreased independently of autophagy. We propose that Vrl1 may regulate autophagy in a similar way as Vps9, and the level of Vrl1 partly decreases through both autophagy-dependent and -independent routes.