Yeast VINE complex recruits phosphatase Glc7 to accelerate Rab5 inactivation, fine-tuning endosomal trafficking.

The precise regulation of Rab5-family GTPases is critical for maintaining endosomal membrane identity and orchestrating protein trafficking. These molecular switches cycle between active (GTP-bound) and inactive (GDP-bound) states, a balance typically controlled by guanine nucleotide exchange factors (GEFs) that activate them, and GTPase-activating proteins (GAPs) that promote inactivation. While VPS9-family GEFs are known to activate Rab proteins at endosomes, the mechanisms integrating positive and negative Rab regulation to fine-tune signaling remain poorly understood, representing a significant gap in our understanding of fundamental cellular processes.

Researchers investigated the role of the yeast VINE complex, a VPS9-family GEF, in regulating the Rab5 homolog Vps21. They employed a multi-pronged approach including genome-wide proximity screening to identify interacting partners, predictive modeling to hypothesize interaction mechanisms, targeted mutagenesis to confirm specific protein domains, and various in vivo assays within yeast cells. This comprehensive strategy aimed to dissect the molecular cascade by which VINE influences Vps21 activity, moving beyond its known role as a GEF to uncover novel regulatory functions.

The study revealed an unexpected role for the VINE complex in promoting the inactivation of the Rab5 homolog Vps21. They found that VINE recruits the protein phosphatase Glc7 through the ankyrin repeat-containing domain of its GEF subunit, Vrl1. This recruitment is crucial for directing the dephosphorylation of Kxd1, a key subunit of the GAP adaptor BLOC-1. The dephosphorylation of Kxd1 subsequently enhances its interaction with Msb3, which is a Vps21-specific GAP. This enhanced interaction ultimately accelerates the GAP-mediated Vps21 inactivation. > The VINE complex, a known Rab activator, paradoxically functions to selectively limit endosomal Rab signaling by recruiting a phosphatase that enhances GAP activity, revealing a novel mechanism of integrated Rab regulation.