Human topoisomerase I C-terminal domain fragment containing the active site tyrosine is a molten globule: implication for the formation of competent productive complex.
Human topoisomerase I (topo I) is an essential cellular enzyme that relaxes DNA supercoiling. The 6.3 kDa C-terminal domain of topo I contains the active site tyrosine (Tyr723) but lacks enzymatic activity by itself. Activity can be fully reconstituted when the C-terminal domain is associated with the 56 kDa core domain. Even though several crystal structures of topo I/DNA complexes are available, crystal structures of the free topo I protein or its individual domain fragments have been difficult to obtain. In this report we analyze the human topo I C-terminal domain structure using a variety of biophysical methods. Our results indicate that this fragment protein (topo6.3) appears to be in a molten globule state. It appears to have a native-like tertiary fold that contains a large population of alpha-helix secondary structure and extensive surface hydrophobic regions. Topo6.3 is known to be readily activated with the association of the topo I core domain, and the molten globule state of topo6.3 is likely to be an energy-favorable conformation for the free topo I C-terminal domain protein. The structural fluctuation and plasticity may represent an efficient mechanism in the topo I functional pathway, where the flexibility aids in the complementary association with the core domain and in the formation of a fully productive topo I complex.
Punchihewa
Chandanamali
C
College of Pharmacy, The University of Arizona, 1703 E. Mabel Street, Tucson, AZ 85721, USA.
Dai
Jixun
J
Carver
Megan
M
Yang
Danzhou
D
D001665:Binding Sites; D004264:DNA Topoisomerases, Type I / Q000737:chemistry*; D006801:Humans; D057927:Hydrophobic and Hydrophilic Interactions; D010446:Peptide Fragments / Q000737:chemistry; D018583:Pliability; D017434:Protein Structure, Tertiary; D014443:Tyrosine
D010446:Peptide Fragments; D014443:Tyrosine; D004264:DNA Topoisomerases, Type I