Mediates the formation of lipid droplets during the differentiation and maturation of adipocytes (PubMed:19278620, PubMed:21533227, PubMed:27564575, PubMed:30293840, PubMed:31708432). Together with LDAF1, defines the sites of lipid droplet formation in the endoplasmic reticulum and promotes the aggregation of triacylglycerol molecules to form the core of a lipid droplet (nucleation) in the endoplasmic reticulum and drive budding of lipid droplets into the cytoplasm (PubMed:31708432). BSCL2 and LDAF1 also regulate protein and lipid delivery from the endoplasmic reticulum into growing lipid droplets (PubMed:27879284, PubMed:31178403). During early adipogenesis, BSCL2 inhibits GPAT3 and GPAT4 activity, thereby preventing phosphatidic acid accumulation that could antagonize PPARG and block differentiation (By similarity). This inhibition is then relieved by ADIG during adipocyte maturation, allowing expansion of lipid droplets (By similarity). Regulates the maturation of ZFYVE1- positive nascent lipid droplets and the function of the RAB18-ZFYVE1 complex in mediating the formation of endoplasmic reticulum-lipid droplet contacts (PubMed:30970241). Binds anionic phospholipids including phosphatidic acid (PubMed:30293840). {ECO:0000250|UniProtKB:Q9Z2E9, ECO:0000269|PubMed:19278620, ECO:0000269|PubMed:21533227, ECO:0000269|PubMed:27564575, ECO:0000269|PubMed:27879284, ECO:0000269|PubMed:30293840, ECO:0000269|PubMed:30970241, ECO:0000269|PubMed:31178403, ECO:0000269|PubMed:31708432}. This is the function of BSCL2 (BSCL2 lipid droplet biogenesis associated, seipin, Ensembl gene identifier ENSG00000168000).