Molecular adapter that stimulates membrane curvature formation and subsequent endoplasmic reticulum exit site (ERES) establishment by recruiting PI3K complex I, leading to COPII vesicle- mediated transport (PubMed:32690950). Promotes endoplasmic reticulum (ER) exit of cGAMP-activated STING1 oligomers (PubMed:32690950, PubMed:37832545). {ECO:0000269|PubMed:32690950, ECO:0000269|PubMed:37832545}. This is the function of STEEP1 (STING1 ER exit protein 1, Ensembl gene identifier ENSG00000018610).