Crystal structure of the calcium-loaded spherulin 3a dimer sheds light on the evolution of the eye lens beta gamma-crystallin domain fold

Background: The beta gamma -crystallins belong to a superfamily of two-doma in proteins found in vertebrate eye lenses, with distant relatives occurrin g in microorganisms. It has been considered that an eukaryotic stress prote in, spherulin 3a, from the slime mold Physarum polycephalum shares a common one-domain ancestor with crystallins, similar to the one-domain 3-D struct ure determined by NMR. Results: The X-ray structure of spherulin 3a shows it to be a tight homodim er, which is consistent with ultracentrifugation studies. The (two-motif) d omain fold contains a pair of calcium binding sites very similar to those f ound in a two-domain prokaryotic beta gamma -crystallin fold family member, Protein S. Domain pairing in the spherulin 3a dimer is two-fold symmetric, but quite different in character from the pseudo-two-fold pairing of domai ns in beta gamma -crystallins. There is no evidence that the spherulin 3a s ingle domain can fold independently of its partner domain, a feature that m ay be related to the absence of a tyrosine corner. Conclusion: Although it is accepted that the vertebrate two-domain beta gam ma -crystallins evolved from a common one-domain ancestor, the mycetezoan s ingle-domain spherulin 3a, with its unique mode of domain pairing, is likel y to be an evolutionary offshoot, perhaps from as far back as the one-motif ancestral stage. The spherulin 3a protomer stability appears to be depende nt on domain pairing. Spherulin-like domain sequences that are found within bacterial proteins associated with virulence are likely to bind calcium.