Responsible, at least in part, for anchoring of the major outer membrane lipoprotein (Lpp, also known as the Braun lipoprotein) to the peptidoglycan via a meso-diaminopimelyl-L-Lys- bond on the terminal residue of Lpp. Cell wall biogenesis; peptidoglycan biosynthesis. Interacts with DsbG. Simultaneous disruption of erfK, ybiS, ycfS and ynhG leads to loss of covalent anchoring of the major outer membrane lipoprotein (Lpp, also known as the Braun lipoprotein) to the peptidoglycan. Complementation with erfK restores some of this anchoring. Belongs to the YkuD family.