Braun's lipoprotein (BLP, Lpp, murein lipoprotein, or major outer membrane lipoprotein) was first identified by V. Braun and K. Rehn in 1969, it was the first Lipoprotein identified prompting much further study in this area. It is found in some
gram-negative
Gram-negative bacteria are bacteria that, unlike gram-positive bacteria, do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. Their defining characteristic is that their cell envelope consists ...
cell walls, is one of the most abundant
membrane proteins; its molecular weight is about 7.2
kDa. It is bound at its C-terminal end (a
lysine) by a
covalent bond to the
peptidoglycan
Peptidoglycan or murein is a unique large macromolecule, a polysaccharide, consisting of sugars and amino acids that forms a mesh-like layer (sacculus) that surrounds the bacterial cytoplasmic membrane. The sugar component consists of alternating ...
layer (specifically to
diaminopimelic acid molecules) and is embedded in the outer membrane by its
hydrophobic
In chemistry, hydrophobicity is the chemical property of a molecule (called a hydrophobe) that is seemingly repelled from a mass of water. In contrast, hydrophiles are attracted to water.
Hydrophobic molecules tend to be nonpolar and, thu ...
head (a
cysteine with lipids attached). BLP tightly links the two layers and provides structural integrity to the outer membrane.
Characteristics
The gene encoding Braun's lipoprotein initially produces a protein composed of 78 amino acids, which includes a 20 amino acid
signal peptide
A signal peptide (sometimes referred to as signal sequence, targeting signal, localization signal, localization sequence, transit peptide, leader sequence or leader peptide) is a short peptide (usually 16–30 amino acids long) present at the ...
at the amino terminus.
The mature protein is 6 kDa in size.
Three monomers of Lpp assemble into a
leucine zipper
A leucine zipper (or leucine scissors) is a common three-dimensional structural motif in proteins. They were first described by Landschulz and collaborators in 1988 when they found that an enhancer binding protein had a very characteristic 30-amin ...
coiled-coil trimer.
Large amounts of Braun's lipoprotein is present, more than any other protein in ''E. coli''.
Unlike other lipoproteins, it is linked covalently to the peptidoglycan.
[ Lpp connects the outer membrane to the peptidoglycan. Lpp is anchored to the outer membrane by its amino-terminal lipid group. In ''E. coli'', one third of Lpp proteins form a peptide bond via the side chain of its carboxy-terminal lysine with diaminopimelic acid in the peptidoglycan layer.][
]
Functions
Lpp, along with another OmpA-like lipoprotein called Pal/OprL (), maintains the stability of the cell envelope
The cell envelope comprises the inner cell membrane and the cell wall of a bacterium. In Gram-negative bacteria an bacterial outer membrane, outer membrane is also included. This envelope is not present in the Mollicutes where the cell wall is abse ...
by attaching the outer membrane to the cell wall.[
Lpp has been proposed as a virulence factor of '' Yersinia pestis'', the cause of plague.]macrophages
Macrophages (; abbreviated MPhi, φ, MΦ or MP) are a type of white blood cell of the innate immune system that engulf and digest pathogens, such as cancer cells, microbes, cellular debris and foreign substances, which do not have proteins that ...
and to efficiently kill mouse models of bubonic and pneumonic plague.
Immunology
Braun's lipoprotein binds to the pattern recognition receptor TLR2. Lpp induces adhesion of neutrophils to human endothelial cells by activating the latter.
References
{{Reflist
Lipoproteins
Peripheral membrane proteins