Lpp, the Braun lipoprotein, turns 50—major achievements and remaining issues

The discovery of Escherichia coli Lpp as the first protein with three acyl groups covalently attached to its N-terminal cysteine residue defined a new class of bacterial proteins, the lipoproteins. Lipoproteins are extracytoplasmic, globular proteins that are anchored to a membrane by a lipid moiety. Being anchored to the outer membrane, Lpp, which is also known as the Braun lipoprotein, is small (5.8 kDa) and folds into a trimeric helical structure. It is also the numerically most abundant protein in E. coli. A unique feature of Lpp is that its C-terminal lysine residue is covalently attached to the peptidoglycan, providing the only covalent connection between the outer membrane and the cell wall. Here, we review the knowledge gained on Lpp since its discovery in 1969 until the recent finding that Lpp functions as a major size determinant in the bacterial cell envelope. We also discuss the role played by Lpp in virulence and highlight the major questions that remain to be solved.