Transports both UDP-glucuronic acid (UDP-GlcA) and UDP-N-acetylgalactosamine (UDP-GalNAc) from the cytoplasm into the endoplasmic reticulum lumen. Plays a role in chondroitin sulfate biosynthesis, which is important for formation of cartilage extracellular matrix and normal skeletal development. Neonatal lethality. Skeletal development is severely impaired leading to reduced snout and body length, and extremely short limbs. The proliferating zone of epiphyseal cartilage is disorganized with densely packed round chondrocytes and little extracellular matrix, in contrast to the regular columnar organization of chondrocytes in wild type cartilage. Chondroitin sulfate content of cartilage is significantly reduced, associated with reduced proteoglycan aggregates in the extracellular matrix. Belongs to the TPT transporter family. SLC35D subfamily. It is uncertain whether Met-1 or Met-31 is the initiator.