Precursors of select GPI-anchored proteins positively regulate GPI biosynthesis under the control of ERAD

We previously reported that glycosylphosphatidylinositol (GPI) biosynthesis is regulated by endoplasmic reticulum associated degradation (ERAD); however, the underlying mechanistic basis remains unclear. Based on a genome-wide CRISPR–Cas9 screen, we show that a widely expressed GPI-anchored protein CD55 precursor and ER-resident ARV1 together upregulate GPI biosynthesis under ERAD-deficient conditions. In cells defective in GPI transamidase, GPI-anchored protein precursors fail to obtain GPI, remaining the uncleaved GPI-attachment signal at the C-termini. We show that ERAD deficiency causes accumulation of the CD55 precursor, which in turn upregulates GPI biosynthesis, where the GPI-attachment signal peptide is the active element. Among the 32 GPI-anchored proteins tested, only the GPI-attachment signal peptides of CD55 and CD48 enhance GPI biosynthesis. ARV1 is essential for the GPI upregulation by CD55 precursor. Our data demonstrate an ARV1-dependent regulatory connection between GPI biosynthesis and precursors of select GPI-anchored proteins that are under the control of ERAD.