The inactivating factor of glutamine synthetase, IF7, is a "natively unfolded" protein

Glutamine synthetase (GS) is the key enzyme responsible for the primary assimilation of ammonium in all living organisms, and it catalyses the synthesis of glutamine from glutamic acid, ATP, and ammonium. One of the recently discovered mechanisms of GS regulation involves protein-protein interactions with a small 65-residue-long protein named IF7. Here, we study the structure and stability of IF7 and its binding properties to GS, by using several biophysical techniques (fluorescence, circular dichroism, Fourier transform infrared and nuclear magnetic resonance spectroscopies, and gel filtration chromatography) which provide complementary structural information. The findings show that IF7 has a small amount of residual secondary structure, but lacks a well defined tertiary structure, and is not compact. Thus, all of the studies indicate that IF7 is a "natively unfolded" protein. The binding of IF7 to GS, its natural binding partner, occurs with an apparent dissociation constant of K(D) = 0.3 +/- 0.1 microM, as measured by fluorescence. We discuss the implications for the GS regulation mechanisms of IF7 being unfolded.