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. Belongs to the GtrA family.