Ca2+-loaded spherulin 3a from Physarum polycephalum adopts the prototype gamma-crystallin fold in aqueous solution

Spherulin 3a is the most abundantly expressed cytosolic protein in spherulating plasmodia of the slime mold Physarum polycephalum. High yields of unlabeled, uniformly 15N and uniformly 13C/15N-labeled recombinant spherulin 3a from Escherichia coli could be produced by a simple protocol described here. The three-dimensional solution structure of Ca2+-loaded spherulin 3a was determined by homo- and heteronuclear NMR spectroscopy. The structure of monomeric spherulin 3a consists of two pleated beta-sheets plus a short alpha-helix arranged into the gamma-crystallin fold. The beta-sheets comprise two intertwined Greek-key motifs. An additional N-terminal beta-strand is unique to spherulin 3a. Complexation of calcium ions greatly enhances overall conformational stability of the protein. The average atomic root-mean-square deviations (r.m.s.d.) for heavy atoms in beta-strands were 0.34(+/-0.16) A for the backbone atoms and 0.73(+/-0.40) A for all atoms. The corresponding r.m.s.d. values for heavy atoms in the whole protein were 0.62(+/-0.42) A for the backbone atoms and 0.99(+/-0.65) A for all atoms. We show the structural relationship between spherulin 3a, a myxomycete dormancy protein, and crystallins from the vertebrate eye lens. Since spherulin 3a has a structure corresponding to one domain of bovine gammaB(II)-crystallin, it represents a hypothetical ancestral gamma-crystallin precursor structure.