Lipoprotein of Gram-negative bacteria is essential for growth and division

Abstract

THE lipoprotein of Gram-negative bacteria has become a cell surface component of central interest following its characterisation by Braun and coworkers¹. This small protein (molecular weight 7,500) is unique in containing as its N-terminal amino acid a glycerylcysteine to which three fatty acids are covalently bound². It is also unusual in its subcellular distribution: one-third of the total lipoprotein is covalently linked to the cell wall peptidoglycan while the remaining two-thirds exists in an unlinked or “free” form in the outer membrane⁴. Free and murein-linked lipoprotein seem to be identical in structure^5,6, and pulse–chase experiments indicate that free lipoprotein is the precursor to murein-linked lipoprotein⁴. Braun's lipoprotein or an immunologically related protein has been found in a wide variety of Gram-negative organisms^7,8, and in Escherichia coli is present in about 3 × 10⁵ copies per cell¹. These observations suggest that lipoprotein performs some essential function(s); however, the nature of that function is unknown. It has been suggested that lipoprotein may have a role in anchoring the outer membrane to the cell wall³, or function in transporting small molecules through the outer membrane⁹ or be involved in cell division^10,11. To clarify the role of Braun's lipoprotein, we have isolated a mutant deficient in its synthesis.