This page largely focuses on brauns lipoprotein but it does have some applicable points for lipoproteins specifically in synthesis :) Braun's [[Lipoprotein]] aka Lpp was the first lipoprotein discovered and is the most abundant protein found in *E. coli*. Not all gram-negative bacteria have this lipoprotein with variations in structure and abundance especially outside of Gammaproteobacteria. ### **Structure of Braun's lipoprotein** Braun's lipoprotein is 8kDa in size and the gene encodes 78 amino acids, it adopts an alpha helical trimeric coiled coil (consisting of 3 Lpp monomers). The lipid portion of Lpp is generally triacylglycerol with three acyl chains and is connected to the peptidic portion at the N-terminus. It contains a C-terminal lysine that binds covalently to the peptidoglycan and this cross link is catalysed by L,D-transpeptidases specifically to mDAP. The lysine is required for this tethering function, although not much investigation has been done as to why, it is just seen that is absence results in a similar phenotype to that of the Lpp deficient strains. **DpaA** is a amidase that can "break" the amide bond between the peptidoglycan and lipoprotein, this is important to modulate flexibility of the cell and facilitate peptidoglycan remodelling. As state above Lpp forms a trimer but only one is covalently linked to the PtG. ### **Biosynthesis** Biosynthesis begins with translation of the Pre[[prolipoprotein]] at the ribosome, where it is then chaperoned by SecB through the periplasm to be translocated through the inner membrane via SecYEG or in some cases the Tat pathway (see [[Protein insertion and translocation]]). It will then be embedded in the inner membrane. Inner membrane protein (IMP) **Lgt** catalyses the attachment of a diacylglycerol moiety from [[Phosphatidylglycerol]] to the thiol group of the conserved N-terminal [[+1]] cysteine on the prolipoprotein via a [[thioether bond]] A second IMP prolipoprotein signal peptidase **LspA** will then remove the N-terminal signal peptide down to the modified [[+1]] cysteine In the context of gram negative bacteria there is a third IMP lipoprotein _N_-acyl transferase **Lnt** which catalyses the addition of a third acyl chain to the amino group of the n-Terminal cysteine The product of these 3 enzymes can then be extracted from the im by LolCDE ATP-Binding-Cassette transported and chaperoned across the periplasm by LolA and outermembrane Lipoprotein LolB **LolCDE** This forms the inner membrane complex. It regulates transport of OM destined and mature lipoproteins for export, most of this is poorly understood but what is seen experimentally is that **lnt** is required for growth, but over expression of LolCDE can alleviate the impact that lnt Ko has on growth, indicating that the third acyl chain my be involved in regulation (Narita and Tokuda, 2011) and something that is only seen in *E. coli* (and other related enterobacteria..) is that removal of the +2 aspartate residue results in lipoproteins accumulating at the inner membrane, but this behaviour is not conserved. (Okuda and Tokuda, 2011) LolC and LolE form the transmembrane domains and are homologous to one another, they interact with one LolD each in the cytosol. The complex contains 4 trans-membrane (two from LolC and two from LolE) then the two LolD monomers function as the nucleotide binding domain for ATP binding and hydrolysis. The overall architecture is similar to MacB type VII ABC transporters LolCE also have substantial periplasmic domains which do differ- the periplasmic domain of LolC forms a hook like structure, and the LolE periplasmic domain is much longer but its structure is vital for function. **LolA** is a periplasmic protein that binds to the periplasmic domain of LolC. There is a significant conformational change upon ATP binding to both **LolD** monomers (it also causes the two LolD NBDs to dimerise) that causes a large enough shift to push lipoproteins out the complex after they have entered. LolA likely has a conformational change upon interaction with a lipoprotein that releases it from the LolC and allows it to move across the periplasm to the outer membrane where it can localise to the OM anchored Lipoprotein **LolB** **LolB** accepts the lipoprotein from LolA and inserts it into the inner leaflet of the OM (Sharma et al., 2021) ![[jb.00149-21-f0001.jpg]] ### **The role of brauns lipoproteins** Lpp functions as a covalent tether for the OM- connecting it to the cell wall. It acts to keep the OM at a fixed distance from the cell wall although there are other tethering proteins (OmpA and Tol-Pal) Lpp is the only covalent linkage and is pervasive across all of the membrane (except the poles, I will discuss this below); both because it is likely less restricted by the low rate of diffusion seen in OM proteins due to its lipid moiety but also because it is the MOST abundant (lipo)protein in bacteria; It is however seen using [[Atomic force microscopy]] that Lpp is not localised to where Pal localises to, which means it is generally excluded from the poles; this is not due to the behaviour of Lpp itself but instead the behaviour of Pal- it is seen that in *pal* KO Lpp is localised all around the cell. LPP deficient cells can grow normally in lab conditions, however are sensitive to hydrophobic antibiotics, chelating agents and toxic compounds such as SDS and EDTA indicating that it contributes to the unique properties of the OM . Lpp deficient cells also release periplasmic proteins into the extracellular medium and bleb the OM providing evidence for its role in maintaining the structural integrity of the cell envelope. Interestingly (and annoyingly) There has been no studies looking at periplasmic width in relation to an *lpp* knock out, however studies have been done looking at extending the length of Lpp, and what was observed was an increased periplasmic width correlated to the length of Lpp. This implies that an *lpp* KO could result in increased periplasmic space but I can not say for sure. Studies have been done looking at the impact of of *lpp* KO on membrane integrity and it is seen that the membrane becomes softer using [[Atomic force microscopy]] **references** - [Asmar, A.T. and Collet, J.-F. (2018). Lpp, the Braun lipoprotein, turns 50—major achievements and remaining issues. _FEMS Microbiology Letters_, 365(18). doi:https://doi.org/10.1093/femsle/fny199.](https://academic.oup.com/femsle/article/365/18/fny199/5071948) - [Diao, J., Komura, R., Sano, T., Pantua, H., Storek, K.M., Inaba, H., Ogawa, H., Noland, C.L., Peng, Y., Gloor, S.L., Yan, D., Kang, J., Katakam, A.K., Volny, M., Liu, P., Nickerson, N.N., Sandoval, W., Austin, C.D., Murray, J. and Rutherford, S.T. (2021). Inhibition of Escherichia coli Lipoprotein Diacylglyceryl Transferase Is Insensitive to Resistance Caused by Deletion of Braun’s Lipoprotein. _Journal of Bacteriology_, 203(13). doi:https://doi.org/10.1128/jb.00149-21.](https://journals.asm.org/doi/10.1128/jb.00149-21) - [Mathelié-Guinlet, M., Asmar, A.T., Collet, J.-F. and Dufrêne, Y.F. (2020). Lipoprotein Lpp regulates the mechanical properties of the E. coli cell envelope. _Nature Communications_, [online] 11(1), pp.1–11. doi:https://doi.org/10.1038/s41467-020-15489-1.](https://www.nature.com/articles/s41467-020-15489-1) - [Narita, S. and Tokuda, H. (2011). Overexpression of LolCDE Allows Deletion of the Escherichia coli Gene Encoding Apolipoprotein _N_ -Acyltransferase. _Journal of Bacteriology_, 193(18), pp.4832–4840. doi:https://doi.org/10.1128/jb.05013-11.](https://journals.asm.org/doi/10.1128/jb.05013-11) - [Okuda, S. and Tokuda, H. (2011). Lipoprotein sorting in bacteria. _Annual Review of Microbiology_, [online] 65, pp.239–259. doi:https://doi.org/10.1146/annurev-micro-090110-102859.](https://doi.org/10.1146%2Fannurev-micro-090110-102859) - [Sharma, S., Zhou, R., Wan, L., Feng, S., Song, K., Xu, C., Li, Y. and Liao, M. (2021). Mechanism of LolCDE as a molecular extruder of bacterial triacylated lipoproteins. _Nature Communications_, [online] 12(1), p.4687. doi:https://doi.org/10.1038/s41467-021-24965-1.](https://www.nature.com/articles/s41467-021-24965-1)