Transient Alpha-helical Structure during Folding of Src SH3 Domain at Subzero Temperatures

Substantial questions remain about the process of protein folding, including whether transient intermediates exist with significantly different secondary structure than the final fold. The src SH3 domain is a highly beta-rich structure with five betastrands. We report here a far-UV circular dichroism investigation of the refolding of His-tagged Src SH3 at subzero temperatures. We observed a transient a-helix-rich intermediate, indicating that the early stages of protein folding can involve the formation of intermediates with very different structures from the final conformation.