GnT1IP-L interacts with medial-Golgi but not trans-Golgi enzymes. (A) Myc-GnT1IP-L or (B) Myc-GnT1IP-S were coexpressed with GlcNAcT-I-HA, GlcNAcT-III-HA, ManIIx-HA, β4GalT-I lacking 13 N-terminal amino acids (SGT-HA), or aa 321–600 of the p85 subunit of PI3 kinase (HA-p85ni) in CHO cells. Lysates were immunoprecipitated with anti-Myc beads, and 40% of the beads were analyzed by immunoblotting on two blots (IP-Myc with IB-HA or IB-Myc). Input (1/40th lysate) was immunoblotted with IB-HA or IB-Myc. (C) GlcNAcT-I-Myc was coexpressed with HA-GnT1IP-L, GlcNAcT-III-HA, ManIIx-HA, or SGT-HA in CHO cells and lysates were treated as in A. ▲, Fragment of GlcNAcT-III-HA not bound by GnT1IP; #, position or predicted position of HA-p85ni; *, immunoglobulin light chains from anti-Myc beads show equal concentration of beads.