sentence: These observations raise the possibility that by targeting lone or unassembled subunits , the Asi complex is involved in the quality control of protein complex assembly.To test this hypothesis , we analyzed the oligosaccharyl transferase ( OST ; Figure 1A ) and the glycosylphosphatidylinositol transamidase ( GPI-T ; Figure S1A ) complexes , which are ER membrane protein complexes that are required for N-linked protein glycosylation and protein GPI anchor attachment , respectively ( Benghezal et al ., 1996 , Fraering et al ., 2001 , te Heesen et al ., 1993 , Kelleher and Gilmore , 1994 ) .