Required for arabinosylation of arabinogalactan (AG), an essential component of the mycobacterial cell wall. Probably acts as an anchor protein recruiting AftA, the first arabinosyl transferase involved in AG biosynthesis. Cell wall biogenesis; cell wall polysaccharide biosynthesis. Interacts with the priming arabinosyltransferase AftA (PubMed:26369580). Also interacts with the galactosyltransferase GlfT1, which initiates the polymerization of the galactan domain of AG (PubMed:23038254). Is thus probably part of an AG biosynthetic complex (PubMed:23038254, PubMed:26369580). Is cotranscribed with dprE1, dprE2 and aftA. Cells lacking this gene display impaired growth and abnormal cell morphology, since they are shorter and more swollen than wild-type cells. This phenotype likely stems from the decreased incorporation of arabinose into arabinogalactan observed in the disruption mutant. The mutant also accumulates decaprenyl-phospho-arabinose (DPA), the arabinose donor required for the synthesis of cell-wall arabinans. The lipoglycan fraction comprising LAM (lipoarabinomannan) and LM (lipomannan) is not affected. Belongs to the GtrA family. Was originally thought to be a lipid-linked sugar translocase involved in the reorientation and export of decaprenyl-phospho-arabinose (DPA) across the plasma membrane to the site of AG (arabinogalactan) and LAM (lipoarabinomannan) synthesis (PubMed:23038254). It was later shown that synthesis of DPA takes place in the periplasm and it was suggested that Rv3789 does not act as a DPA flippase but, rather, recruits AftA for arabinogalactan biosynthesis (PubMed:25906160, PubMed:26369580).