Cryo-EM structures of human GMPPA-GMPPB complex reveal how cells maintain GDP-mannose homeostasis

Structural elucidation and functional analysis of the human GMPPA-GMPPB complex reveals how GMPPA acts as a 'sensor' of GDP-mannose to allosterically regulate GMPPB activity. GDP-mannose (GDP-Man) is a key metabolite essential for protein glycosylation and glycophosphatidylinositol anchor synthesis, and aberrant cellular GDP-Man levels have been associated with multiple human diseases. How cells maintain homeostasis of GDP-Man is unknown. Here, we report the cryo-EM structures of human GMPPA-GMPPB complex, the protein machinery responsible for GDP-Man synthesis, in complex with GDP-Man or GTP. Unexpectedly, we find that the catalytically inactive subunit GMPPA displays a much higher affinity to GDP-Man than the active subunit GMPPB and, subsequently, inhibits the catalytic activity of GMPPB through a unique C-terminal loop of GMPPA. Importantly, disruption of the interactions between GMPPA and GMPPB or the binding of GDP-Man to GMPPA in zebrafish leads to abnormal brain development and muscle abnormality, analogous to phenotypes observed in individuals carrying GMPPA or GMPPB mutations. We conclude that GMPPA acts as a cellular sensor to maintain mannose homeostasis through allosterically regulating GMPPB.

Automated summary

The structural elucidation and functional analysis of the human GMPPA-GMPPB complex demonstrates how GMPPA acts as a 'sensor' to regulate GMPPB activity and maintain cellular GDP-mannose homeostasis through allosteric mechanisms. This study reveals that GMPPA has a higher affinity to GDP-mannose compared to GMPPB, and can inhibit the catalytic activity of GMPPB. Disruption of GMPPA-GMPPB interactions or GDP-mannose binding to GMPPA leads to abnormal development phenotypes in zebrafish, similar to individuals with GMPPA or GMPPB mutations.