Critical role of WW domain phosphorylation in regulating phosphoserine binding activity and Pin1 function

Phosphoserine-binding modules help determine the specificity of signal transduction events. One such module, the group IV WW domain, plays an essential role in targeting the phosphorylation-specific prolyl isomerase Pin 1 to its substrates. These modules require Ser/Thr phosphorylation of their ligands for binding activity. However, phosphorylation of these modules and its functional significance have not been described, nor is it known whether the function of Pin 1 is regulated. Here we show that Pin 1 WW domain is phosphorylated on Ser(16) both in vitro and in vivo. Further, this phosphorylation regulates the ability of the WW domain to mediate Pin 1 substrate interaction and cellular localization. Moreover, both Pin 1 and WW domain mutants refractory to Ser(16) phosphorylation act as dominant-negative mutants to induce mitotic block and apoptosis and increase multinucleated cells with 8 N DNA content. Thus, phosphorylation is a new mechanism critical for regulating WW domain phosphoserine binding activity and Pinl function.