Lipid transporter involved in lipid countertransport between the endoplasmic reticulum and the plasma membrane: specifically exchanges phosphatidylserine with phosphatidylinositol 4-phosphate (PI4P), delivering phosphatidylserine to the plasma membrane in exchange for PI4P, which is degraded by the SAC1/SACM1L phosphatase in the endoplasmic reticulum. Binds phosphatidylserine and PI4P in a mutually exclusive manner (PubMed:26206935). Binds oxysterol, 25-hydroxycholesterol and cholesterol (PubMed:17428193, PubMed:17991739, PubMed:21698267). Interacts with SPAG5 (PubMed:24424245). Interacts with NUP62 (PubMed:21698267). The presence of the N-terminus extension contains an overall negative charge that may explain the weak localization to the cortical endoplasmic reticulum (Probable). Localizes to the cortical endoplasmic reticulum at the endoplasmic reticulum-plasma membrane contact sites. Widely expressed (PubMed:11735225). Expressed at higher level in macrophages (PubMed:17991739). Belongs to the OSBP family.