The sialylation of glycoconjugates is performed by a variety of sialyltransferases using CMP-sialic acid (CMP-Sia) as a substrate. Sialylation requires the translocation of CMP-Sia across the Golgi membranes. This function has been assigned to SLC35A1, the only CMP-Sia transporter identified to date. Mutations in the SLC35A1 gene cause a subtype of congenital disorder of glycosylation (CDG). […] We showed that SLC35A1 associates with ST3Gal4, the main α2,3-sialyltransferase acting on N-glycans. This phenomenon is compromised by the E196K (but not T156R) mutation in the SLC35A1 gene. We also demonstrated that the E196K mutant is less efficient in restoring N-glycan sialylation upon expression in the SLC35A1 knockout cells.