The antigen 85 proteins (FbpA, FbpB, FbpC) are responsible for the high affinity of mycobacteria for fibronectin, a large adhesive glycoprotein, which facilitates the attachment of Mycobacteria to murine alveolar macrophages (AMs) (PubMed:8406884). They also help to maintain the integrity of the cell wall by catalyzing the transfer of mycolic acids to cell wall arabinogalactan and through the synthesis of alpha,alpha-trehalose dimycolate (TDM, cord factor). They catalyze the transfer of a mycoloyl residue from one molecule of alpha,alpha-trehalose monomycolate (TMM) to another TMM, leading to the formation of TDM (By similarity). 2 alpha,alpha'-trehalose 6-mycolate = alpha,alpha'-trehalose 6,6'-bismycolate + alpha,alpha-trehalose a 1,2-diacyl-sn-glycerol + an acyl-CoA = a triacyl-sn-glycerol + CoA Three regions of this protein interact with the collagen-binding domain of human fibronectin (FN1); binding is saturable and competed by gelatin (which binds to the same domain) but not by heparin (which binds to another domain of fibronectin) (PubMed:8406884). Belongs to the mycobacterial A85 antigen family.