Proteins have many distinct tertiary folds (Richardson, J. S. (1981) A
dv. Prot. Chem. 34, 167-339). The term tertiary fold refers to the spa
tial organization of secondary structure elements (alpha-helices and b
eta-strands). It is not known when, in the process of protein folding,
a native tertiary fold emerges. Here, we show that the helical domain
of human alpha-lactalbumin, in isolation, forms a molten globule with
the same overall tertiary fold as that found in intact alpha-lactalbu
min. Formation of this nativelike fold does not require extensive, spe
cific side-chain packing. Our results suggest that much of the informa
tion transfer from one-dimension to three-dimensions has occurred at t
he molten globule stage of protein folding.