GnTIP-L inhibition of GlcNAcT-I is specific and deletion mutants do not inhibit. (A) CHO cells stably expressing various GnT1IP-L constructs were tested for L-PHA resistance. Asterisk signifies that ∼50% cells survived at 70 µg/ml L-PHA. (B) CHO cells stably expressing Myc-GnT1IP-L after hygromycin selection (Myc-GnT1IP-L^Hyg) were sorted for GNA binding (Myc-GnT1IP-L^GNA) and analyzed by flow cytometry. Shaded profiles are autofluorescence. Lysates were analyzed by immunoblot (IB Myc), stripped and reprobed with anti-actin mAb. (C) GlcNAcT-I and β4GalT activities in lysates from CHO cells stably expressing vector or GNA-sorted GnT1IP-L^GNA or Myc-GnT1IP-L^GNA or HA-GnT1IP-L^GNA (GNA-sorted twice). Bars represent range (n = 2). (D) Lysates from Lec1 cells stably expressing GlcNAcT-I-HA and transiently expressing GFP or Myc-GnT1IP-S were assayed for GlcNAcT-I and β4GalT activities. Bars represent range of duplicates. Myc-GnT1IP-S and GlcNAcT-I-HA levels were determined by immunoblotting with actin as loading control. (E) GlcNAcT-I, GlcNAcT-III, and β4GalT specific activities (SA) in lysates from CHO and LEC10 cells stably expressing control vector or the GnT1IP construct shown. GnT1IP transfectants were GNA-sorted. Bars represent range (n = 2).