Together with LptE, is involved in the assembly of lipopolysaccharide (LPS) at the surface of the outer membrane. Component of the lipopolysaccharide transport and assembly complex. Interacts with LptE and LptA. Non-essential under typical in vitro growth conditions. Loss of lptD causes impaired in vitro growth. Decreases LPS levels and susceptibility to polymyxin B, increases outer membrane permeability and hypersensitivity to hydrophobic antibiotics. Causes an accumulation of lipid IV(A). Deletion impairs cell envelope integrity more than the loss of LPS biosynthesis, presumably due to the accumulation of toxic intermediates. Belongs to the LptD family.