May serve as a regulatory subunit and allow allosteric feedback inhibition of GMPPB by GDP-mannose. 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.