SLOW ALPHA-HELIX FORMATION DURING FOLDING OF A MEMBRANE-PROTEIN

Very little is known about the folding of proteins within biological m embranes. A ''two-stage'' model has been proposed on thermodynamic gro unds for the folding of ct helical, integral membrane proteins, the fi rst stage of which involves formation of transmembrane a helices that are proposed to behave as autonomous folding domains. Here, we investi gate ct helix formation in bacteriorhodopsin and present a time-resolv ed circular dichroism study of the slow ia vitro folding of this prote in. We show that, although some of the protein's a helices form early, a significant part sf the protein's secondary structure appears to fo rm late in the folding process. Over 30 amino acids, equivalent to at least one of bacteriorhodopsin's seven transmembrane segments, slowly fold from disordered structures to ct helices with an apparent rate co nstant of about 0.012 s(-1) at pH 6 or 0.0077 s(-1) at pH 8. This is a rate-limiting step in protein folding, which is dependent on the pH a nd the composition of the lipid bilayer.