Divalent cations in native and reaggregated mycoplasma membranes

Area of Science:

• Microbiology
• Biochemistry
• Membrane Biology

Background:

• Mycoplasma and Acholeplasma are wall-less bacteria with unique membrane compositions.
• Divalent cations like magnesium (Mg2+) play crucial roles in membrane structure and function.
• Understanding cation binding is essential for elucidating membrane organization.

Purpose of the Study:

• To quantify Mg2+ content in Mycoplasma and Acholeplasma membranes.
• To investigate the binding sites of Mg2+ within these membranes.
• To explore the impact of membrane reaggregation on Mg2+ content and localization.

Main Methods:

• Atomic absorption spectroscopy for Mg2+ quantification.
• Lipid extraction using chloroform-methanol.
• Pronase digestion to assess protein-bound Mg2+.
• Dialysis and membrane reaggregation techniques.
• Ethylenediaminetetraacetic acid (EDTA) chelation assays.

Main Results:

• Mg2+ content varied significantly (0.88–1.98 µg/mg protein) across species and growth conditions.
• Isolated Acholeplasma laidlawii membranes showed increased Mg2+ upon dialysis and reaggregation.
• 60–83% of Mg2+ was associated with the lipid fraction, primarily phospholipids.
• Pronase digestion minimally affected Mg2+ content, indicating limited protein binding.
• EDTA released most, but not all, Mg2+, revealing a tightly bound fraction.

Conclusions:

• Mg2+ in Mycoplasma and Acholeplasma membranes is predominantly bound to lipids, especially phospholipids.
• Membrane reaggregation alters Mg2+ content and suggests changes in molecular organization.
• A small, constant pool of Mg2+ remains tightly bound, irrespective of chelation.