THE CHAPERONIN GROEL DOES NOT RECOGNIZE APO-ALPHA-LACTALBUMIN IN THE MOLTEN GLOBULE STATE

We investigate here the interaction between GroEL and two kinds of non -native alpha-lactalbumin. alpha-lactalbumin is a Ca2+-binding protein which assumes a molten globule state in the absence of Ca2+ (apo-alph a-lactalbumin) at neutral pH. Our results, obtained by molecular-sieve chromatography and hydrogen-exchange measurements, show that apo-alph a-lactalbumin in this molten globule state is not bound to GroEL eithe r in the absence or in the presence of KCl. On the other hand, we show by molecular-sieve chromatography that alpha-lactalbumin, in which th e four disulphide bonds are fully reduced, is bound to GroEL when 50 m M KCl is present. The results demonstrate that the protein state recog nized by GroEL is more unfolded and expanded than the typical molten g lobule state of alpha-lactalbumin.