Golgi targeting of Drosophila melanogaster β4GalNAcTB requires a DHHC protein family-related protein as a pilot

Drosophila melanogaster beta 4GalNAcTB mutant flies revealed that this particular N-acetylgalactosaminyltransferase is predominant in the formation of lacdiNAc (GalNAc beta 1,4GlcNAc)-modified glycolipids, but enzymatic activity could not be confirmed for the cloned enzyme. Using a heterologous expression cloning approach, we isolated beta 4GalNAcTB together with beta 4GalNAcTB pilot (GABPI), a multimembrane-spanning protein related to Asp-His-His-Cys (DHHC) proteins but lacking the DHHC consensus sequence. In the absence of GABPI, inactive beta 4GalNAcTB is trapped in the endoplasmic reticulum ( ER). Coexpression of beta 4GalNAcTB and GABPI generates the active enzyme that is localized together with GABPI in the Golgi. GABPI associates with beta 4GalNAcTB and, when expressed with an ER retention signal, holds active beta 4GalNAcTB in the ER. Importantly, treatment of isolated membrane vesicles with Triton X-100 disturbs beta 4GalNAcTB activity. This phenomenon occurs with multimembrane-spanning glycosyltransferases but is normally not a property of glycosyltransferases with one membrane anchor. In summary, our data provide evidence that GABPI is required for ER export and activity of beta 4GalNAcTB.