| First Author | Ury B | Year | 2021 |
| Journal | J Biol Chem | Volume | 296 |
| Pages | 100789 | PubMed ID | 34015330 |
| Mgi Jnum | J:325957 | Mgi Id | MGI:6713130 |
| Doi | 10.1016/j.jbc.2021.100789 | Citation | Ury B, et al. (2021) The promiscuous binding pocket of SLC35A1 ensures redundant transport of CDP-ribitol to the Golgi. J Biol Chem :100789 |
| abstractText | The glycoprotein alpha-dystroglycan helps to link the intracellular cytoskeleton to the extracellular matrix. A unique glycan structure attached to this protein is required for its interaction with extracellular matrix proteins such as laminin. Up to now, this is the only mammalian glycan known to contain ribitolphosphate groups. Enzymes in the Golgi apparatus use cytidine diphosphate (CDP)-ribitol to incorporate ribitolphosphate into the glycan chain of alpha-dystroglycan. Since CDP-ribitol is synthesized in the cytoplasm, we hypothesized that an unknown transporter must be required for its import into the Golgi apparatus. We discovered that CDP-ribitol transport relies on the CMP-sialic acid transporter SLC35A1 and the transporter SLC35A4 in a redundant manner. These two transporters are closely related, but bulky residues in the predicted binding pocket of SLC35A4 limit its size. We hypothesized that the large binding pocket SLC35A1 might accommodate the bulky CMP-sialic acid and the smaller CDP-ribitol, whereas SLC35A4 might only accept CDP-ribitol. To test this, we expressed SLC35A1 with mutations in its binding pocket in SLC35A1 knockout cell lines. When we restricted the binding site of SLC35A1 by introducing the bulky residues present in SLC35A4, the mutant transporter was unable to support sialylation of proteins in cells, but still supported ribitolphosphorylation. This demonstrates that the size of the binding pocket determines the substrate specificity of SLC35A1, allowing a variety of cytosine nucleotide conjugates to be transported. The redundancy with SLC35A4 also explains why patients with SLC35A1 mutations do not show symptoms of alpha-dystroglycan deficiency. |
