HELIX-LOOP-HELIX, HELIX-TURN-HELIX
- 1. HELIX- TURN- HELIX & HELIX- LOOP- HELIX BIOTECHNOLOGY - BY- NAREN YADAV
- 2. DNA – Binding Domain A DNA-binding domain (DBD) is an independently folded protein domain that contains at least one motif that recognizes double- or single-stranded DNA. A DBD can recognize a specific DNA sequence (a recognition sequence) or have a general affinity to DNA. 2
- 3. Types of DNA-Binding Motifs • 1. Steroid Receptor • 2. Zinc Fingers • 3. Leucine Zipper • 4. Homeodomain • 5.HELIX-TURN-HELIX • 6.HELIX-LOOP-HELIX 3
- 4. Helix-turn-helix • In proteins, the helix-turn-helix (HTH) is a major structural motif capable of binding DNA. 4
- 5. DISCOVERY • The discovery of the helix-turn-helix motif was based on similarities between several genes encoding transcription regulatory proteins from bacteriophage lambda and Escherichia coli: Cro, CAP, and λ repressor, which were found to share a common 20-25 amino acid sequence that facilitates DNA recognition. 5
- 6. EXAMPLE 6 The λ repressor of bacteriophage lambda employs a helix-turn-helix (left; green) to bindDNA (right; blue and red).
- 7. STRUCTURE • The structure consist of two α helices joined by a short strand of amino acid. • The recognition and binding to DNA by helix-turn-helix proteins is done by the two α helices, one occupying the N- terminal end of the motif, the other at the C-terminus. • In most cases, such as in the Cro repressor, the second helix contributes most to DNA recognition, and hence it is often called the "recognition helix". 7
- 8. Helix-turn-helix C-terminal binds to major groove, N-terminal helps to position the complex 8
- 9. FUNCTION The helix-turn-helix (HTH) is a major structural motif capable of binding DNA. It is found in many proteins that regulate gene expression. It binds to the major groove of DNA through a series of hydrogen bonds and various Van der Waals interactions with exposed bases. The other α helix stabilizes the interaction between protein and DNA, but does not play a particularly strong role in its recognition. The recognition helix and its preceding helix always have the same relative orientation. 9
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- 11. CLASSIFICATION OF HELIX TURN HELIX Di-helical Tri-helical Tetra-helical Winged helix-turn-helix Other modified helix-turn-helix motifs 11
- 12. Helix-turn-helix review LOCATION •Bacterial regulatory proteins, •Related motifs in eukaryotic proteins. CHARACTERIST -ICS •Two alpha helices BINDING SITE IN DNA •Major groove 12
- 13. Helix-loop-helix - The helix-loop-helix binding motif consists of two alpha helices separated by a loop of amino acids. 13
- 14. STRUCTURE -Two polypeptide chains with the motif join to form a functional DNA-binding protein. - A highly basic set of amino acids in one of the helices binds to the DNA. - Transcription factor including this domain are in dimeric form. 14
- 15. CONTINUE… • In general, one helix is smaller, and, due to the flexibility of the loop, allows dimerization by folding and packing against another helix. • The larger helix typically contains the DNA-binding regions. • bHLH proteins typically bind to a consensus sequence called an E-box, CANNTG. 15
- 16. FUNCTION 16 bHLH transcription factors are often important in development or cell activity. BMAL1-Clock is a core transcription complex in the molecular circadian clock. Other genes, like c-Myc and HIF-1, have been linked to cancer due to their effects on cell growth and metabolism.
- 17. REGULATION • Since many bHLH transcription factors are heterodimeric, their activity is often highly regulated by the dimerization of the subunits. • One subunit's expression or availability is often controlled, whereas the other subunit is constitutively expressed. 17
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- 19. Helix-loop-helix review LOCATION •Eukaryotic protein CHARACTERIST -ICS •Two alpha helices separated by a loop of amino acids. BINDING SITE IN DNA •Major groove 19
- 20. EXAMPLES • AhR,BMAL-1-CLOCK,C-Myc, • N-Myc,MyoD,Myf5,Scleraxis, • Neurogenis, etc… 20
- 21. •THANK YOU 21