May serve as a regulatory subunit and allow allosteric feedback inhibition of GMPPB by GDP-mannose. Associates with GMPPB. Widely expressed. Belongs to the transferase hexapeptide repeat family. GMPPA is a close homolog of GMPPB, that has been shown to catalyze the formation of GDP-mannose, an essential precursor of glycan moieties of glycoproteins and glycolipids. It has been hypothesized that GMPPA might serve as a regulatory subunit and allow allosteric feedback inhibition of GMPPB by GDP-mannose. Alignment of GMPPAs and GMPPBs from various species shows that GMPPAs are characterized by a 2 amino acid-insertion (residues 11-12) in a highly conserved motif that borders the catalytic pocket and binds the nucleotide substrate in homologous enzymes. This insertion might inactivate the ancestral catalytic site, converting it to an allosteric site (PubMed:24035193).