Compositions and methods involving engineered P27

A trimeric protein complex comprising:

• (i) a polypeptide comprising an engineered p27, wherein the engineered p27 comprises amino acid substitutions at positions Y74, Y88, and Y89, wherein the amino acid substitution at position Y74 is Y74E, Y74D, or Y74R, the amino acid substitution at Y88 is Y88E or Y88D, and the amino acid substitution at Y89 is Y89E or Y89D, wherein the amino acid positions are determined with reference to the sequence of SEQ ID NO:
• 1, and wherein the engineered p27 has at least 90% sequence identity to SEQ ID NO:
• 1;
• (ii) a Cdk4 or a variant thereof, or a Cdk6 or a variant thereof;
• and
• (iii) a CycD or a variant thereof,
• wherein the Cdk4 or the variant thereof or the Cdk6 or the variant thereof in the trimeric protein complex is an active kinase.