Low-resolution structure of the soluble domain GPAA1 (yGPAA170-247) of the glycosylphosphatidylinositol transamidase subunit GPAA1 from Saccharomyces cerevisiae.

The GPI (glycosylphosphatidylinositol) transamidase complex catalyses the attachment of GPI anchors to eukaryotic proteins in the lumen of ER (endoplasmic reticulum). The Saccharomyces cerevisiae GPI transamidase complex consists of the subunits yPIG-K (Gpi8p), yPIG-S (Gpi17p), yPIG-T (Gpi16p), yPIG-U (CDC91/GAB1) and yGPAA1. We present the production of the two recombinant proteins yGPAA1⁷⁰⁻²⁴⁷ and yGPAA1⁷⁰⁻³³⁹ of the luminal domain of S. cerevisiae GPAA1, covering the amino acids 70-247 and 70-339 respectively. The secondary structural content of the stable and monodisperse yGPAA1⁷⁰⁻²⁴⁷ has been determined to be 28% α-helix and 27% β-sheet. SAXS (small-angle X-ray scattering) data showed that yGPAA1⁷⁰⁻²⁴⁷ has an R(g) (radius of gyration) of 2.72±0.025 nm and D(max) (maximum dimension) of 9.14 nm. These data enabled the determination of the two domain low-resolution solution structure of yGPAA1⁷⁰⁻²⁴⁷. The large elliptical shape of yGPAA1⁷⁰⁻²⁴⁷ is connected via a short stalk to the smaller hook-like domain of 0.8 nm in length and 3.5 nm in width. The topological arrangement of yGPAA1⁷⁰⁻²⁴⁷ will be discussed together with the recently determined low-resolution structures of yPIG-K²⁴⁻³³⁷ and yPIG-S³⁸⁻⁴⁶⁷ from S. cerevisiae in the GPI transamidase complex.