Details of Sr2+ binding by Pex5p(C). A. Elemental analysis of Pex5p(C) – the upper panel shows the Rutherford backscattering spectrum used for determining the gross matrix composition and thickness of the sample. The lower panel shows the PIXE spectrum, horizontal bars indicating the characteristic emission bands for sulphur (which can be seen) and strontium (where there are no counts). The insets compare the sulphur and strontium elemental PIXE maps from the drop of protein. B. Stereoview of the Sr2+ binding site in the crystal structure. Residues 479–486 are shown as sticks, the backbone oxygen atoms of Val479, Asp482 and Ser485 coordinating the Sr2+ ion. Three water molecules also visibly coordinate the Sr2+. C. A superposition of the Pex5p Sr2+-binding loop with the subtilisin CATMAT4 motif; the similarity was discovered searching the MSDmotif database . Residues 482–485 of the loop in Pex5p(C) superimpose perfectly on the Ca-binding site of subtilisin. Pex5p is shown in cyan (carbon atoms and waters) and green (strontium) colours, and subtilisin in orange (carbon atoms and water) and magenta (calcium).