We investigated the amino acid sequences of rat PERIOD2 (rPER2) that are 
required for interaction with CRYPTOCHROME1 (CRY1) to understand the molecular 
mechanism of the circadian clock. Co-immunoprecipitation assays using various 
C-terminal fragments of rPER2 with internal deletions revealed that amino acid 
residues 1179-1198 are necessary for interaction with CRY1. To identify 
precisely which amino acid residues are responsible for the interaction, we 
substituted alanine for residues conserved among PER isoforms and species. We 
found that more than three mutations of conserved PER2 residues impaired not 
only binding to CRY1 but also subsequent nuclear translocation, although 
mutations of non-conserved residues did not affect interaction with CRY1. Thus, 
the conserved amino acid residues of 1179-1198 in PER2 are apparently 
responsible for binding to CRY1.