A single point mutation changes the crystallization behavior of Mycoplasma arthritidis-derived mitogen

Area of Science:

• Structural Biology
• Protein Crystallography
• Immunology

Background:

• Mycoplasma arthritidis-derived mitogen (MAM) is a superantigen involved in immune responses.
• Previous attempts to crystallize wild-type MAM (MAMwt) resulted in poor diffraction (5.0 A resolution) and large unit cells, suggesting multiple molecules in the asymmetric unit.

Purpose of the Study:

• To obtain higher quality crystals of MAM for improved structural analysis.
• To identify specific mutations that enhance crystal formation and diffraction properties.

Main Methods:

• Systematic alanine mutagenesis was employed to create MAM mutants.
• Crystallization was performed using the hanging-drop vapour-diffusion method.
• X-ray diffraction data were collected and analyzed, including molecular replacement calculations.

Main Results:

• Two mutants, L50A and K201A, were successfully crystallized.
• Crystals of L50A were isomorphous with MAMwt, while K201A formed a new cubic crystal form.
• Diffraction data to 3.6 A (L50A) and 2.8 A (K201A) resolution were obtained.
• An interpretable electron-density map was generated for the K201A mutant using molecular replacement.

Conclusions:

• Alanine mutagenesis is an effective strategy for improving crystal quality of MAM.
• The K201A mutant yielded significantly better diffraction, facilitating detailed structural studies.
• The improved crystal structure of MAM K201A provides insights into its superantigen function.