Involved in the biosynthesis of lipid A, a phosphorylated glycolipid that in bacteria anchors the lipopolysaccharide to the outer membrane of the cell. Transfers the gamma-phosphate of ATP to the 4'-position of a tetraacyldisaccharide 1-phosphate intermediate (termed DS-1-P) to form tetraacyldisaccharide 1,4'-bis-phosphate (lipid IVA). Lipid A-like molecules in plants may serve as structural components of the outer membranes of mitochondria and/or chloroplasts, or may be involved in signal transduction or plant defense responses (Potential). ATP + {2-N,3-O-bis[(3R)-3-hydroxytetradecanoyl]-beta-D-glucosaminyl}-(1->6)-{2-N,3-O-bis[(3R)-3-hydroxytetradecanoyl]-alpha-D-glucosaminyl phosphate} = ADP + {2-N,3-O-bis[(3R)-3-hydroxytetradecanoyl]-4-O-phospho-beta-D-glucosaminyl}-(1->6)-{2-N,3-O-bis[(3R)-3-hydroxytetradecanoyl]-alpha-D-glucosaminyl phosphate}. Glycolipid biosynthesis; lipid IV(A) biosynthesis; lipid IV(A) from (3R)-3-hydroxytetradecanoyl-[acyl-carrier-protein] and UDP-N-acetyl-alpha-D-glucosamine: step 6/6. A number of isoforms are produced. According to EST sequences. No visible phenotype under normal growth conditions, but plants lacking LPXK accumulate high levels of disaccharide-1-phosphate (DS-1-P). Belongs to the LpxK family.