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. Binds oxysterol, 25-hydroxycholesterol and cholesterol. Interacts with SPAG5. Interacts with NUP62. The presence of the N-terminus extension contains an overall negative charge that may explain the weak localization to the cortical endoplasmic reticulum. Widely expressed. Most abundant in liver, spleen, kidney, brain and adipose tissue. Mice are apparently healthy and show no developmental defects nor gross abnormality of glucose or lipid metabolism. However, they display a significant elevation of plasma HDL cholesterol and phospholipid levels. They also show a modest increase of APOA1, plus a number of mild gender- or diet-specific lipid metabolism phenotypes (PubMed:23554939). Deletion in bone marrow-derived cells, including macrophages, reduces atherosclerotic lesion progression (PubMed:25347070). Belongs to the OSBP family.