Strep-tag II for one-step affinity purification of active bHLHzip domain of human c-Myc.

The c-Myc protein, the product of the c-myc protooncogene, is a nuclear phosphoprotein with DNA-binding properties when heterodimerized with the Max protein. It contains an amino-terminal transcriptional activation domain and a carboxy-terminal basic helix-loop-helix leucine zipper (bHLHzip) domain that directs heterodimerization and promotes DNA binding. Here, we describe the isolation of the bHLHzip domain of human c-Myc with a technique for efficient single-step purification. Using a C-terminal Strep-tag II affinity peptide and a novel Streptactin-Sepharose matrix, elution is performed under mild conditions by competition with the biotin analog desthiobiotin. No significant influence of the affinity tag on the activity of the bHLHzip domain was observed when the fusion protein was subjected to glutathione S-transferase (GST) pull-down assays for investigating its in vitro-binding properties with GST-Max. The use of the C-terminal Strep-tag II was shown to be more suitable for obtaining pure product fractions than use of the N-terminal GST affinity tag.