SOS3 (salt overly sensitive 3) from Arabidopsis thaliana:: expression, purification, crystallization and preliminary X-ray analysis

The salt-tolerance gene SOS3 ( salt overly sensitive 3) of Arabidopsis thaliana encodes a calcium-binding protein that is able to sense the cytosolic calcium signal elicited by salt stress. SOS3 activates the SOS2 protein kinase, which activates various ion transporters. SOS3 was cloned into a plasmid and expressed in Escherichia coli, allowing purification of the protein to homogeneity. Two crystals with different additive contents were grown. Both diffract to 3.2 Angstrom resolution and belong to space group I4(1), with unit-cell parameters a = 93.65, c = 80.08 Angstrom and a = 91.79, c = 85.78 Angstrom, respectively. A promising molecular-replacement solution has been found using neuronal calcium-sensor 1 as the search model. Interestingly, no solution was found using AtCBL2 ( A. thaliana calcineurin B-like protein) structure as a search model, although this protein belongs to the same family and displays 50% sequence identity.