AbstractLipopolysaccharides (LPS) are central components of the outer membrane and consist of Lipid A, the core polysaccharide, and the O-antigen. The synthesis of LPS is initiated at the cytosolic face of the cytoplasmic membrane. The subsequent transport to and across the outer membrane involves multiple lipopolysaccharide transport (Lpt) proteins. Among those proteins, the periplasmic-localized LptA and the outer membrane-embedded LptD participate in the last steps of transfer and insertion of LPS into the outer membrane. While the process is described for proteobacterial model systems, not much is known about the machinery in cyanobacteria. We demonstrate thatanaLptD (alr1278) ofAnabaenasp. PCC 7120 is important for cell wall function and its pore domain shows a Lipid A sensitive cation-selective gating behavior. The N-terminal domain ofanaLptD recognizesanaLptA (alr4067), but notecLptA. Furthermore,anaLptA specifically interacts with the Lipid A fromAnabaenasp. PCC 7120 only, whileanaLptD binds to Lipid A isolated fromEscherichia colias well. Based on the comparative analysis of proteins fromE. coliandAnabaenasp. we discuss the properties of the cyanobacterial Lpt system.