Catalyzes the formation of GDP-mannose, an essential precursor of glycan moieties of glycoproteins and glycolipids. alpha-D-mannose 1-phosphate + GTP + H(+) = diphosphate + GDP-alpha-D-mannose Nucleotide-sugar biosynthesis; GDP-alpha-D-mannose biosynthesis; GDP-alpha-D-mannose from alpha-D-mannose 1-phosphate (GTP route): step 1/1. Expressed throughout all stages of development. Morpholino knockdown of the gene results in smaller embryos with multiple anomalies, including bent tails, hypopigmentation, microphthalmia, hydrocephalus, and reduced motility. Muscle fibers in mutant animals are sparse and disorganized, and the myosepta are damaged or incompletely developed. There is also evidence of sarcolemmal damage. Immunostaining shows defective glycosylation of DAG1 associated with abnormal structure of the basement membrane. Belongs to the transferase hexapeptide repeat family.