[Show abstract][Hide abstract] ABSTRACT: Src SH3 is a small all-beta-sheet protein composed of a single domain. We studied the folding behavior of src SH3 at various conditions by circular dichroism (CD), fluorescence, and X-ray solution scattering methods. On the src SH3 folding pathway, an alpha-helix-rich intermediate appeared not only at subzero temperatures but also above 0 degrees C. The fraction of alpha-helix in the kinetically observed intermediate is ca. 26% based on the kinetic CD experiment. X-ray solution scattering revealed that the intermediate was compact but not fully packed. The analysis of CD implies that the amplitude of the burst phase is proportional to the helical fraction calculated according to the helix-coil transition theory. This strongly suggests that the initial folding core is formed by the collapse of much less stably existing alpha-helices.