Abstract
Cardiolipin (CL) is a universal component of energy generating membranes. In most bacteria, it is synthesized via the condensation of two molecules phosphatidylglycerol (PG) by phospholipase D-type cardiolipin synthases (PLD-type Cls). In the plant pathogen and natural genetic engineer Agrobacterium tumefaciens CL comprises up to 15% of all phospholipids in late stationary growth phase. A. tumefaciens harbors two genes, atu1630 (cls1) and atu2486 (cls2), coding for PLD-type Cls. Heterologous expression of either cls1 or cls2 in Escherichia coli resulted in accumulation of CL supporting involvement of their products in CL synthesis. Expression of cls1 and cls2 in A. tumefaciens is constitutive and irrespective of the growth phase. Membrane lipid profiling of A. tumefaciens mutants suggested that Cls2 is required for CL synthesis at early exponential growth whereas both Cls equally contribute to CL production at later growth stages. Contrary to many bacteria, which suffer from CL depletion, A. tumefaciens tolerates large changes in CL content since the CL-deficient cls1/cls2 double mutant showed no apparent defects in growth, stress tolerance, motility, biofilm formation, UV-stress and tumor formation on plants.
Highlights
Bacterial cytoplasmic membranes consist of various lipids with distinct chemical and physical properties
Using the protein sequences of well characterized cardiolipin synthases (Cls) of the phospholipase D (PLD)- and CAP-type as query, we identified two open reading frames on the circular chromosome of A. tumefaciens, encoding PLD-type CL synthases (Cls) homologs
The deduced Cls1 and Cls2 proteins display only about 23% sequence identity whereas Cls1 is about 41% identical to ClsC from E. coli (Fig 1A)
Summary
Bacterial cytoplasmic membranes consist of various lipids with distinct chemical and physical properties. They are arranged in a two-dimensional matrix with proteins embedded, allowing selective transport, sensing, communication and energy-generating processes. The most abundant prokaryotic membrane lipids are the glycerophospholipids phosphatidylethanolamine (PE), phosphatidylglycerol (PG) and cardiolipin (CL). CL is an anionic phospholipid with varying amounts in bacterial membranes. Despite its low abundance during exponential growth, CL influences diverse physiological processes, such as localization and stability of proteins and protein complexes, formation of membrane microdomains and the generation of membrane potential [1,2,3,4,5]. Due to its four acyl chains and a small hydrophilic headgroup, CL displays a cone-shaped architecture and preferably locates at regions of negative membrane curvature [6,7,8]
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