Calcium ion binding by tobacco mosaic virus

Area of Science:

• Biophysics
• Biochemistry
• Virology

Background:

• Tobacco mosaic virus (TMV) is a well-characterized plant virus.
• Understanding ion binding is crucial for viral stability and interactions.

Purpose of the Study:

• To quantify calcium ion binding to tobacco mosaic virus.
• To investigate the influence of pH and ionic strength on calcium binding.

Main Methods:

• Calcium ion titrations using a calcium-specific ion-exchange electrode.
• Scatchard analysis to determine binding site number and affinity.
• Experiments conducted in water and 0.01 M KCl solutions.

Main Results:

• TMV possesses two distinct calcium ion binding sites per protein subunit with high affinity (stability constants > 10(4) M-1).
• Calcium binding releases hydrogen ions, with up to two H+ released per Ca2+ at higher affinity sites in water.
• Apparent stability constants and hydrogen ion release vary with pH and ionic strength.

Conclusions:

• The electrical charge on the TMV particle may remain relatively constant across a pH range despite expected variations.
• Independent titration of two non-identical binding sites explains observed calcium-ion interactions.
• These findings provide insights into the electrostatic properties and ion-binding behavior of TMV.