Antigen 85B is the most abundant protein expressed by Mycobacterium tuberculosis (about one quarter). It is a myocolyc transferase in the myc pathway and catalyses - like Ag85A and Ag85C - the transfer of the fatty acid mycolate from one trehalose monomycolate to another, resulting in trehalose dimycolate and free trehalose and helping build the cell wall.
Proteins of the antigen 85 complex are responsible for the high affinity of mycobacteria to fibronectin. Possesses a mycolyltransferase activity required for the biogenesis of trehalose dimycolate (cord factor), a dominant structure necessary for maintaining cell wall integrity.