S100 proteins modulate p53 activity by interacting with its tetramerization 
(p53TET, residues 325-355) and transactivation (residues 1-57) domains. In this 
study, we characterized biophysically the binding of S100A1, S100A2, S100A4, 
S100A6 and S100B to homologous domains of p63 and p73 in vitro by fluorescence 
anisotropy, analytical ultracentrifugation and analytical gel filtration. We 
found that S100A1, S100A2, S100A4, S100A6 and S100B proteins bound different p63 
and p73 tetramerization domain variants and naturally occurring isoforms with 
varying affinities in a calcium-dependent manner. Additional interactions were 
observed with peptides derived from the p63 and p73 N-terminal transactivation 
domains. Importantly, S100 proteins bound p63 and p73 with different affinities 
in their different oligomeric states, similarly to the differential modes of 
binding to p53. On the basis of our data, we hypothesize that S100 proteins 
regulate the oligomerization state of all three p53 family members and their 
isoforms, with a potential physiological relevance in developmental and 
disease-related processes. The regulation of the p53 family by S100 is 
complicated and depends on the target preference of each individual S100 
protein, the concentration of the proteins and calcium, as well as the splicing 
variation of p63 or p73. Our results outlining the complexity of the interaction 
should be considered when studying the functional effects of S100 proteins in 
their biological context.