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:23934110, PubMed:26206935). May cooperate with NPC1 to mediate the exit of cholesterol from endosomes/lysosomes (PubMed:21220512). Binds 25-hydroxycholesterol and cholesterol (PubMed:17428193). Localizes to endoplasmic reticulum-plasma membrane contact sites (EPCS). Localizes to the cortical endoplasmic reticulum at the EPCS. Ubiquitously expressed. According to a report, the gene is imprinted in placenta, where it is predominantly expressed from the maternal allele only. Not imprinted in other tissues (PubMed:12504849). According to another report, it is not imprinted in trophoblast stem cells (PubMed:20644730). Belongs to the OSBP family. Contaminating sequence. Sequence of unknown origin in the N-terminal part.