About 150 various proteins are anchored to the mammalian cell surface via a glycolipid, termed glycosylphosphatidylinositol (GPI). GPI is synthesized in the ER and transferred to the proteins as a posttranslational modification, generating GPI-anchored proteins. GPI-anchored proteins then undergo the remodeling of GPI in the ER and the Golgi before arriving at the cell surface. The chemical structure of mammalian GPI was reported 30 years ago. GPI consists of conserved core glycan and inositolphospholipid. In some GPI-anchored proteins, N-acetylgalactosamine (GalNAc) is attached to the first mannose as a side-chain, which can be elongated by galactose and sialic acid. Although we have clarified the most genes in GPI biosynthetic pathway in last 25 years, the biosynthetic pathway of GPI-GalNAc side-chain remains unknown.

In this study, we aimed to identify and characterize the GPI-GalNAc transferase, which catalyzes the first reaction in GPI-GalNAc side-chain biosynthesis. We identified an uncharacterized protein, TMEM246, as a candidate which we renamed post-GPI attachment to proteins 4 (PGAP4). PGAP4 is a Golgi-resident protein with three transmembrane domains and, as our expectation, a GPI-GalNAc transferase. There are many Golgi-resident glycosyltransferases, all of them are single pass type II transmembrane proteins. Therefore, PGAP4 has a unique structure among Golgi glycosyltransferases. 3D structural modeling of PGAP4 suggests a concave surface for accommodation of GPI-glycan at the vicinity of the three transmembrane domains (Figure). From these results, we obtained the mechanistic insight in the GalNAc addition to GPI by PGAP4. The study will help understanding the physiological function of GPI-GalNAc side-chain and the establishment of the novel concept about the Golgi-resident glycosyltransferases.

This work was performed in Dr. Taroh Kinoshita’s laboratory (Yabumoto Department of Intractable Disease Research) in collaboration with Dr. Yoshiki Yamaguchi’s group in RIKEN (Structural Glycobiology Team), Dr. Morihisa Fujita’s group in Jiangnan University (School of Biotechnology), and Dr. Shota Nakamura’s group (Department of Infection Metagenomics) and Dr. Kazunobu Saito’s group (Core Instrumentation Facility) in Research Institute for Microbial Diseases, Osaka University.

Identification of a Golgi GPI-N-acetylgalactosamine transferase with tandem transmembrane regions in the catalytic domain.