p115

p115 is a 115-kilodalton cytosolic tethering factor that operates at the early secretory pathway in eukaryotic cells. In humans the protein is encoded by the USO1 gene and is conserved across diverse species, with homologs such as Uso1p in yeast. p115 functions as a Rab1 effector that facilitates the docking of ER-derived vesicles to the Golgi apparatus, thereby promoting efficient ER-to-Golgi transport.

Structurally, p115 is a coiled-coil protein that forms a dimer and resides predominantly in the cytosol near the ER–Golgi interface. It is recruited to membranes through interactions with Rab1-GTP on transport vesicles and with Golgi surface proteins, enabling it to bridge vesicles and the target membrane.

Functionally, p115 acts as a vesicle tether, capturing transport vesicles at the Golgi and facilitating subsequent SNARE-mediated membrane fusion. By bridging Rab1-bearing vesicles with Golgi membranes, p115 promotes SNARE complex assembly and docking, contributing to the fidelity and efficiency of ER-to-Golgi trafficking. Disruption of p115 function or expression leads to defects in protein trafficking and distortion of Golgi structure, highlighting its essential role in the secretory pathway.

Interactions of p115 include Rab1, Golgi matrix proteins such as GM130 and giantin, and SNAREs such as syntaxin 5, membrin, and Bet1. Through these interactions, p115 coordinates tethering with downstream fusion machinery, acting as a critical intermediary between vesicle formation at the ER and fusion at the Golgi.

p115 is studied as a model component of vesicle tethering and membrane trafficking, illustrating the mechanisms by which cytosolic tethers coordinate vesicle docking with SNARE-dependent fusion in eukaryotic cells.