Unexpectedly, SCRIB and SLC3A2 form a quaternary complex with LLGL2-SLC7A5 to promote membrane assembly of the SLC7A5/SLC3A2 amino acid transporter complex, which is needed for leucine uptake and proliferation of ER+ breast cancer cells in culture and in vivo. The gene discussed is SLC3A2; the disease is breast carcinoma.