Most phospholipids-essential building blocks of cellular membranes-are synthesized in the endoplasmic reticulum (ER) and distributed to the intracellular membranes. Yeast phosphatidylserine (PtdSer) is produced in the ER and is transported to the mitochondria, Golgi, or vacuoles; it is subsequently converted into phosphatidylethanolamine by phosphatidylserine decarboxylase. In yeast, PstB2p (Sec14p homolog) and Pbi1p are known to be involved in non-vesicular lipid transport from the ER to the Golgi, however, the molecular mechanisms remain unclear. In this study, we attempted to analyze the stoichiometric model of the PstB2p-Pbi1p complex in PtdSer transport from the ER to Golgi by using size exclusion chromatography with multi-angle static light scattering, mass spectrometry, reductive methylation, and homology modeling. The homology model of PstB2p was validated in part via reductive methylation, suggesting that it has structure similar to that of Sec14p. We observed that PstB2p forms a homodimer but exists as a 1:1 heterodimer in the presence of Pbi1p. When PtdSer was added to the PstB2p-Pbi1p complex, PtdSer bound to PstB2p, triggering the dissociation of the PstB2p-Pbi1p complex. PstB2p in complex with PtdSer exists as a monomer in contrast to its homodimeric form in the absence of PtdSer. These findings suggest that a stoichiometric model of the PstB2p-Pbi1p complex in yeast can be used to study the PtdSer transport system. (C) 2019 Elsevier Inc. All rights reserved.