Functional Insights on the GPI-GnT Complex involved in the Initial Step of the GPI anchor Biosynthesis

NOGUERA MARTIN E.; D'ALESSIO CECILIA; SANTOS JAVIER; D'AMICO, SAMANTHA DAMIELA

Congreso; LI Reunión Anual de la Sociedad Argentina de Biofísica; 2023

The addition of glycosylphosphatidylinositol (GPI) anchors is an important posttranslational modification that occurs in the endoplasmic reticulum (ER) of eukaryotes that allows GPI-bearing proteins to be attached to the cell surface without the need of a transmembrane domain. This anchoring strategy is used by diverse proteins and failures in GPI synthesis lead to severe or fatal human diseases. GPI biosynthesis is a complex process that takes place in both sides of the ER membrane whose first step is the transfer of GlcNAc from UDP-GlcNAc to phosphatidylinositol. The reaction is catalyzed by the seven-subunits GPI-N-acetylglucosaminyltransferase (GPI-GnT) complex from which PIGA is the catalytic subunit. Despite its biological significance, limited knowledge exists regarding the structural biology and the roles of accessory subunits of GPI-GnT complex. We used a range of bioinformatics methods to identify each subunit orthologue in a largeset of eukaryotic organisms. By analyzing the presence of each subunit in the complex, we detected a hierarchical arrangement were the catalytic subunit PIGA is always present and mainly assisted by the subunits PIGC, PIGP and PIGQ. A combined analysis of sequence conservation and phylogeny led us to the identification of potentially functionally important sequence elements in the complex. To better understand the GPIGnT structure, we used structural models generated by AlphaFold2 to predict ligand binding to the catalytic subunit. Furthermore, by considering data on disease-associated missense mutations, we were able to rationalize the impact of some pathologicalmutations on the structural properties of the complex. Our findings provide new insights into the GPI-GnT structure and mechanisms of disease caused by mutations in the GPIGnT coding genes