Antiporter that transports nucleotide sugars across the endoplasmic reticulum (ER) membrane in exchange for another nucleotide sugar. May couple UDP-alpha-D-glucuronate (UDP-GlcA) or UDP-alpha-D- xylose (UDP-Xyl) efflux to UDP-alpha-D-glucuronate (UDP-GlcA) influx into the ER lumen, which in turn stimulates glucuronidation and excretion of endobiotics and xenobiotics. {ECO:0000269|PubMed:16965264, ECO:0000305|PubMed:15911612}. [Isoform 1]: Has UDP-GlcA:UDP-GlcNAc antiporter activity. . [Isoform 2]: Has UDP-GlcA:UDP-GlcNAc antiporter activity. . This is the function of ENSG00000205060 (SLC35B4, solute carrier family 35 member B4).