# Lipopolysaccharide: Recent Advances in Its Biosynthesis and Controlling Cell Envelope Homeostasis
###### Abstract
Suppressor analysis of \(\Delta\)_lapD_ revealed genetic interactions with _lpxM_, _clsA_ (cardiolipin synthesis), and _acpP_. The interaction of LapD with the acetyl-CoA carboxylase enzyme complex and FASII enzymes and AcpP suggests that LapD could facilitate the coordinated regulation of fatty acid/PL and LPS and the existence of a broader hub. Some supporting evidence for the role of LapD in fatty acid regulation has recently been presented [27]. (iii) New structural insights into LPS assembly protein LapB and its function: As mentioned above, LapB is a multitasking protein that interacts with FtsH to mediate LpxC degradation. However, the mechanism by which LpxC is specifically recognized remains unclear. Recent studies have shown that LapB of _E. coli_ uses its cytoplasmic TPR elements and C-terminal rubredoxin domain to interact with LpxC [28]. Another study in _Salmonella_ showed that TPR2 of LapB facilitates LpxC binding and that the N-terminal transmembrane anchor, TPR1, TPRs 5-9, and the rubredoxin domain are not required for LapB-LpxC interaction under the experimental conditions used in these studies [29]. Thus, more studies are required to identify the specific amino acid residues in LapB that interact with FtsH and LpxC to regulate LPS levels. Interestingly, species-specific variations have also been reported for LapB. For example, in _Ikbeisella pneumoniae_, the LapB structure reveals a distinct open-ring dimeric architecture, generated by TPR motif sliding, in contrast to the closed-ring dimer reported in the case of _E. coli_[30]. In _K. pneumoniae_, structural analysis of the dimeric form revealed interactions primarily via TPR5 of one monomer and TPR9 of the other [30]. (iv) New players in the regulation of LPS, regulatory control by PlsB, and requirement of alarmone ppGpp: Although most of the current studies on the regulation of LPS synthesis have centered around regulatory control via LpxC-FtsH/LapB and LapC, additional research shows that LpxA activity mediating the initial step in lipid A biosynthesis by itself might be one of the points of control. It has been shown that ppGpp inhibits LpxA activity [31]. Adding a new layer to the regulation of LPS, it has been shown that the essential GTPase ObgE interacts with LpxA. This was demonstrated based on the characterization of a dominant negative version of ObgE, which inhibits LpxA activity when bound to GTP [32]. Consistent with this regulation, suppressors that relieve the toxicity of ObgE were mapped to the _lpxA_ gene [32]. A role for ppGpp in LPS biosynthesis was also previously shown in strains constitutively expressing the RpoE sigma factor, since \(\Delta\)_rseA_ ppGpp\({}^{0}\) strains were unable to modify lipid A by Ara4N [19]. It has been known for a long time that the accumulation of ppGpp inhibits both de novo membrane PL biosynthesis and the incorporation of exogenous fatty acids into PL [33]. This inhibition was attributed to the accumulation of long-chain acyl-ACPs, which are substrates for the _plsB_ gene product _sn_-glycerol-3-phosphate acyltransferase. In fatty acid/PL biosynthesis, key regulatory points are mediated by controlling the initiation rate-limiting step mediated by the carboxylation of acetyl-CoA by the ACC complex to produce malonyl-CoA, from which the malonyl group is transferred to _holo_-ACP by FabD to enter the elongation process. The second and probably more important is the regulation of the first step in PL synthesis, catalyzed by the PlsB enzyme, which synthesizes lysophosphatidic acid from long-chain ACP and _sn_-glycerol-3 phosphate. Based on experimental and modeling studies, it has been shown that PL flux is regulated primarily via post-translational control of PlsB enzyme activity [34]. It has been shown that PlsB activity controls the concentration of the LPS precursor C14:0-OH-ACP, linking fluxes into the PL and LPS synthesis pathways, as altering PL synthesis by adjusting either PIsB or ACC activity would also vary LPS synthesis [34].