Involved in the biosynthesis of the arabinogalactan (AG) region of the mycolylarabinogalactan-peptidoglycan (mAGP) complex, an essential component of the mycobacterial cell wall. Catalyzes the addition of the first key arabinofuranosyl (Araf) residue from the sugar donor decaprenyl-phospho-arabinose (DPA) on the C-5 of a 6-linked galactofuranosyl (Galf) of the galactan domain, thus 'priming' the galactan for further elaboration by other arabinofuranosyltransferases. Adds an alpha-D-arabinofuranosyl group from trans,octacis-decaprenylphospho-beta-D-arabinofuranose at the 5-O-position of the eighth, tenth and twelfth galactofuranose unit of the galactofuranan chain of [beta-D-galactofuranosyl-(1->5)-beta-D-galactofuranosyl-(1->6)]14-beta-D-galactofuranosyl-(1->5)-beta-D-galactofuranosyl-(1->4)-alpha-L-rhamnopyranosyl-(1->3)-N-acetyl-alpha-D-glucosaminyl-diphospho-trans,octacis-decaprenol. Cell wall biogenesis; cell wall polysaccharide biosynthesis. Belongs to the glycosyltransferase 85 family.