Mann-Whitney U-tests for the distributions of numerical attributes around protein surface atoms.

<p>The y-axis of matrix shows the atom type index (<i>i</i> = 30 protein atom types shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0037706#pone-0037706-t001" target="_blank">Table 1</a>) and the x-axis shows the <i>j</i> index for the 32 <i>A_i,j</i> features, where <i>j</i> = 1,31 represents the 31 interacting atom types shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0037706#pone-0037706-t001" target="_blank">Table 1</a> and the 32^nd feature reflects the local geometry of the protein surface. The matrix element (<i>j,i</i>) shows the Mann-Whitney U-test p-value in color-code for the two groups of <i>A_i,j</i> : one group of <i>A_i,j</i> was calculated for the attribute type <i>j</i> around the surface atom type <i>i</i> in the known PPI sites on proteins in the S432 dataset and the other group was calculated for the same attribute type around the non-PPI site atom type <i>i</i> in the same dataset. The p-values were calculated with the Mann-Whitney U-test implemented as the function ranksum in MATLAB. Two sets of data were input to the function and the output p-value is the probability for the two distributions of data to be statistically indistinguishable. The plus(+) sign in the matrix element indicates that the averaged feature value for the PPI site atoms is larger than the averaged feature value for the non-PPI site atoms and the negative(−) is the opposite. The panel on the right-hand-side of the matrix shows the distributions of protein surface atoms in PPI sites (blue) and non-PPI protein surfaces (red) against protein atom type. The data were derived from proteins in S432.