Purification, crystallization and preliminary X-ray diffraction of human S100A15

Abstract

Human S100A15 is a novel member of the S100 family of EF-hand calcium-binding proteins and was recently identified in psoriasis, where it is significantly upregulated in lesional skin. The protein is implicated as an effector in calcium-mediated signal transduction pathways. Although its biological function is unclear, the association of the 11.2 kDa S100A15 with psoriasis suggests that it contributes to the pathogenesis of the disease and could provide a molecular target for therapy. To provide insight into the function of S100A15, the protein was crystallized to visualize its structure and to further the understanding of how the many similar calcium-binding mediator proteins in the cell distinguish their cognate target molecules. The S100A15 protein has been cloned, expressed and purified to homogeneity and produced two crystal forms. Crystals of form I are triclinic, with unit-cell parameters a = 33.5, b = 44.3, c = 44.8 angstroms, alpha = 71.2, beta = 68.1, gamma = 67.8 degrees and an estimated two molecules in the asymmetric unit, and diffract to 1.7 angstroms resolution. Crystals of form II are monoclinic, with unit-cell parameters a = 82.1, b = 33.6, c = 52.2 angstroms, beta = 128.2 degrees and an estimated one molecule in the asymmetric unit, and diffract to 2.0 angstroms resolution. This structural analysis of the human S100A15 will further aid in the phylogenic comparison between the other members of the S100 protein family, especially the highly homologous paralog S100A7.

Figure 1

Purification of S100A15 protein. The protein sample was separated by 4–12% SDS–PAGE as shown for the following stages of purification, extraction and IMAC. M, protein standards (kDa); S, starting sample; P, insoluble fraction; L, column load; FT, flowthrough fraction; W, wash fraction. Arrows denote the location of his6-MBP-A15. The solid line indicates the fractions combined to prepare sample pools.

Figure 2

The crystal morphology of the S100A15 protein is shown. No attempt was made to optimize the crystal size and morphology. The initial crystallization screen leads to crystals with the same morphology but two different space groups that coexist in the same drop.