Solution Structure and Calcium-Binding Properties of M-Crystallin, A Primordial βY-Crystallin from Archaea

The lens beta gamma-crystallin superfamily has many diverse but topologically related members belonging to various taxa. Based on structural topology, these proteins are considered to be evolutionarily related to lens crystallins, suggesting their origin from a common ancestor. Proteins with crystallin domains, although found in some eukaryotes and eubacteria, have not yet been reported in archaea. Sequence searches in the genome of the archaebacterium Methanosarcina acetivorans revealed the presence of a protein annotated as a beta gamma-crystallin family protein, named M-crystallin. Solution structure of this protein indicates a typical beta gamma-crystallin fold with a paired Greek-key motif. Among the known structures of beta gamma-crystallin members, M-crystallin was found to be structurally similar to the vertebrate lens beta gamma-crystallins. The Ca2+-binding properties of this primordial protein are somewhat more similar to those of vertebrate beta gamma-ycrystallins than to those of bacterial homologues. These observations, taken together, suggest that amphibian and vertebrate beta gamma-crystallin domains are evolutionarily more related to archaeal homologues than to bacterial homologues. Additionally, identification of a beta gamma-crystallin homologue in archaea allows us to demonstrate the presence of this domain in all the three domains of life. (c) 2008 Elsevier Ltd. All rights reserved.