A tetragonal crystal form of horse spleen apoferritin and its relationship to the cubic modification.

This research examines the tetragonal form of horse spleen apoferritin, a type of protein, comparing its crystal structure to the cubic form and confirming its structural similarity to native molecules.

Key Features of Horse Spleen Apoferritin

• The study focuses on horse spleen apoferritin, a protein that has been crystallized as tetragonal plates and needles.
• A tetragonal crystal shape is one where two axes have the same length and the third axis is different from the other two.
• The horse spleen’s apoferritin tetragonal form has a unit cell with designated measures of a = b = 147 ± 0.5 A and c = 154.4±0.5 A. This indicates the lengths of its sides within the crystal structure.
• The space group of the tetragonal apoferritin is P4212, which is a descriptor of its symmetry characteristics.
• The unit cell has two molecules in a pseudo-body-centred arrangement, meaning the molecules are not centered exactly in the unit cell but are situated in a pattern that resembles such an arrangement.

Comparison with Cubic Crystals

• The intensity distributions and calculated rotation functions of tetragonal and cubic crystals were compared to understand the structural differences and commonalities.
• In the cubic crystals, the molecules followed a 432 symmetry with 4-fold axes lying along the cube axes. This suggests a high level of symmetry in the cubic form.
• In the tetragonal crystals, one molecule’s 4-fold axis is parallel to c, the unique axis.
• The remaining 4-fold axes of the two molecules, located in planes perpendicular to c, are rotated ± 17.5° with respect to the tetragonal a axis. This shows a slight shift in the positioning of the molecules in the tetragonal form compared to the cubic form.

Significance of Findings

• The fact that apoferritin can be reassembled from subunits and crystallized in both tetragonal and cubic forms confirms its conformational similarity to native molecules.
• This understanding of the crystal forms of horse spleen apoferritin and its similarity to the structures of other native molecules could facilitate further studies in protein crystallography and provide insights into protein function and behavior.