The function of GMPPA (GDP-mannose pyrophosphorylase A, ENSG00000144591) is as follows. Regulatory subunit of the GMPPA-GMPPB mannose-1-phosphate guanylyltransferase complex; reduces the catalytic activity of GMPPB when part of the complex (PubMed:24035193, PubMed:33986552). Mediates allosteric feedback inhibition of GMPPB catalytic activity upon binding GDP-alpha-D-mannose (PubMed:24035193, PubMed:33986552). Together with GMPPB regulates GDP-alpha-D-mannose levels (PubMed:33986552). .