# Regulated Assembly of LPS, Its Structural Alterations and Cellular Response to LPS Defects
## 6 Assembly of LPS Requires LapB  
Besides a role for turnover of LpxC in concert with FtsH, LapB has been implicated in the assembly of LPS at the IM presumably along with LapA [9]. Genes encoding _lapA_ and _lapB_ are co-transcribed from three promoters, the distal promoter located upstream of the _pgsB_ gene, the middle promoter recognizedby the RopH heat shock sigma factor and the last one resembling house-keeping promoters [9]. Overall, such a transcriptional organization suggests coupling of transcription with phospholipid metabolism using _pgp_ co-transcription. PgpB encodes phosphatidylglycerophosphatase, an enzyme that is part of the phosphatidylglycerol biosynthesis pathway, which is itself part of phospholipid metabolism. Transcription from the heat shock promoter ensures transcription of _lapA/B_ genes at high temperatures, hence belonging to heat shock regulon, which comprises several chaperones and proteases.  
The evidence that LapB plays a role for LPS assembly in the IM comes from experimental evidence that include: (a) The _lapB_ gene is essential for bacterial growth and a deletion of the _lapB_ gene can be constructed in the presence of suppressor that either restore phospholipid synthesis like _sfhC21_, or decrease the LPS synthesis like in the presence of mutations in an early lipid A biosynthesis pathway (_lpxA_, _lpxC_, _lpxD_) or when LPS is composed mostly of Kdo\({}_{2}\)-lipid A derivatives like (_waaC lapB_) combination [9]. (b) The lack of LapB leads to accumulation of LPS precursor forms. These precursor forms represent pentaacylated lipid A species, the presence of Kdo\({}_{2}\)-dilauroyl-lipid IV\({}_{A}\). (c) \(\Delta\)_lapB_ mutants have defects in the folding of LPS-specific enzymes like LpxM and conserved glycostransferases like WaaC, WaaO. A large proportion of these enzymes is present in aggregate form and hence LPS-specific enzymes like LpxM and core glycosyltransferase could be limiting explaining the accumulation of LPS precursor species [9]. (d) LapB also co-purifies with heptosyltransferase I. (e) LapA/B could function synergistically with classical chaperones like DnaK/DnaJ based on LapA/LapB co-purification with DnaK/J, multicopy suppression of growth defects of \(\Delta\)(_lap lapB_) mutants, even more pronounced defects in the LPS composition when \(\Delta\)(_lapA lapB__dnaK/J_) were examined. (f) LapA/B co-purify with LPS, Lpt proteins and FtsH. (g) A \(\Delta\)(_lapA-lapB_) mutation is synthetically lethal with a compromised LptD variant or when SurA is absent. (h) The absence of LapB induces a strong envelope stress response regulated by RopE, Cpx and Rcs systems by inducing transcription of genes, whose products are required to maintain homeostasis in the cell envelope. LapB contains nine tetraticopeptide repeat (TPR) motifs and a C-terminal rubredoxin domain [9; 37]. Mutations in the either rubredoxin domain or TPR/interface impair cell growth [9]. Thus, based on the essentiality of TPR repeats in LapB, which can mediate protein-protein interactions (interaction of LapA/B with LPS-specific enzymes) a model was presented wherein LapA/B could form a scaffold-like structure for delivery of various acyltransferases and glycostransferases to the site in IM where LPS is assembled and delivered to the Lpt complex. At such an assembly site, proteolysis of LpxC can occur given co-purification of LapA/B with FtsH to prevent excessive LPS buildup. This may serve as an essential purpose of preventing wasteful transfer of incompletely synthesized LPS. This model draws support from the IM association of LpxC in _Neisseria meningitidis_, which requires the presence of LapB counterpart called Ght [38]. Consistent with a broad essential function of LapB, a suppressor mutation _lapBV_43G in the _lapB_ gene that confers protection against prolonged exposure to phosphate starvation and also suppresses the _mlaA_-dependent hyperproduction of LPS have been reported [39; 40]. Thus, LapB plays an important role in maintaining cell envelope homeostasis and assembly of LPS.