lac Repressor cysteine-140 reacts selectively with a fluorescent probe bound to the core-headpiece interface

Area of Science:

• Biochemistry
• Molecular Biology
• Protein Chemistry

Background:

• The lac repressor protein regulates gene expression.
• Specific cysteine residues are targets for chemical modification.
• Fluorescent probes are used to study protein dynamics.

Purpose of the Study:

• To investigate the mechanism of selective reaction of I-AEDANS with Cys-140 of the lac repressor.
• To determine if I-AEDANS binds to the interface between the core and headpiece regions.
• To understand the factors influencing I-AEDANS selectivity.

Main Methods:

• Chemical modification using fluorescent probes (I-AEDANS).
• Enzymatic digestion (trypsin) to create repressor core protein.
• Comparative analysis of I-AEDANS reactivity with intact repressor and core protein.
• Varying ionic strength and pH to assess binding interactions.

Main Results:

• I-AEDANS selectivity for Cys-140 was significantly reduced upon removal of the repressor headpiece.
• Iodoacetamide, lacking the naphthylsulfonate group, showed minimal selectivity.
• Ionic strength influenced I-AEDANS selectivity, indicating non-ionic binding interactions.
• pH changes had a minor effect on selectivity.

Conclusions:

• I-AEDANS selectively reacts with Cys-140 by binding to the interface between the lac repressor core and headpiece regions.
• The naphthylamine sulfonate moiety of I-AEDANS is crucial for this interface binding and subsequent reaction.
• Cys-140 is strategically positioned for fluorescent probes to report on headpiece motions during DNA binding.