Numerous endosomal transcripts were upregulated after GATA-1 activation in G1E erythroid cells. These transcripts included enzymes which modify glycoprotein sugars, proteases, proton pumps and proteins involved in transferrin transport. Several enzymes which modify red cell membrane glycoproteins were induced such as N-acetyl galactosaminidase alpha (Naga) and "protective protein for beta-galactosidase" (Ppbg) a protein which stabilizes the activity of lysosomal galactosidase and neuraminidase; it also has cathepsin A activity. Cathepsin B is upregulated throughout the time course; this enzyme may have a role in destruction of improperly folded proteins or autophagy. Similar upregulation was noted in MEL 1 and K562 2 cells.
Upregulated vacuolar proton pump components (Atp6v0b, Atp6m, Atp6d) may acidify endosomes required for transferrin receptor shuttling. Gabarap, the gamma-aminobutyric acid receptor associated protein was induced in parallel. Gabarap binds to the cytoplasmic domain of transferrin receptor 1 (Trfr), but does not appear to affect transferrin receptor endocytosis. Based on homology to other family members, such as APG8/Aut7 in yeast, Gabarap may target transferrin receptor to a degradative pathway 3. The transcript for transferrin receptor 1 increases abundance up to 21 hours, and then declines.
These histograms represent a time course of transcript signal levels at 0, 3, 7, 14, 21 and 30 hours after activation of GATA-1