May serve as a regulatory subunit and allow allosteric feedback inhibition of GMPPB by GDP-mannose. Associates with GMPPB. Myc-tagged GMPPA shows a diffuse cytoplasmic and nuclear pattern in transfected COS-7 cells. Expressed in fibroblasts (at protein level). The disease is caused by variants affecting the gene represented in this entry. 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. However, lymphocytes from AAMR patients, that exhibit very low GMPPA protein levels, have unchanged GDP-mannose pyrophosphorylase activity and higher GDP-mannose levels than those from healthy controls. Affected individuals and control subjects show similar N-glycosylation profiles, both for transferrin glycosylation and for N-glycans derived from either total serum protein or immunoglobulin G. These observations led to the hypothesis 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).