Involved in the import of GDP-mannose from the cytoplasm into the Golgi lumen. Defective copy causes severe glycosylation defect and abnormal retention of soluble endoplasmic reticulum proteins. Involved in vanadate sensitivity. Homooligomer. Recycles between the Golgi apparatus and the endoplasmic reticulum. Present with 31900 molecules/cell in log phase SD medium. Belongs to the TPT transporter family. SLC35D subfamily.