Synthetic corticotropin (adrenocorticotropic hormone)-releasing factor [CRF; for the sequence, see Vale, W., Spiess, J., Rivier, C. & Rivier, J. (1981) Science 213, 1394Science 213, -1397 in aqueous solution exists predominantly as a random coil. At concentrations greater than 1 ,uM, the peptide shows a tendency to self-aggregate with a concurrent slight increase in the apparent a-helical content as measured by the CD spectrum. The a-helix formed by this molecule is highly amphiphilic-i.e., the hydrophilic and hydrophobic regions are segregated on opposite faces of the helix. As predicted from the potential amphiphilic structure, CRF binds avidly to the surface of single bilayer egg phosphatidylcholine vesicles. This binding appears to obey a simple Langmuir isotherm with the following parameters: Kd = 1.3 + 0.6 x 10-7 M and capacity at saturation (N) = 11.0 -1.0 mmol of peptide per mol of phospholipid. CRF also readily forms an insoluble monolayer at the air-water interface. The monolayer is composed of monomers of the hormone with molecular areas, A' = 22 A2 per amino acid, suggesting a compact secondary structure. judged from the collapse pressure (19.0 ± 0.1 dyne/cm; 1 dyne = 10 ,uN) of the monolayer, the amphiphilicity of CRF approximates that of plasma apolipoproteins, a class of proteins of the most pronounced amphiphilic character. These results suggest that the binding of CRF to the cell membrane is accompanied by the induction of an ahelical secondary structure and it is this predominantly helical form that is the biologically active form of the peptide.Corticotropin (adrenocorticotropic hormone)-releasing factor (CRF) is a hormone produced by the hypothalamus; it increases the rate of secretion of corticotropin by the pituitary gland (1, 2). The primary structure of ovine CRF has been determined by Vale et al. (3) and is shown in Fig. 1. Vale et al. also reported that a considerable portion of CRF is highly homologous to sauvagine, a peptide isolated from the skin of the South American frog Phylomedusa sauvagei and to urotensin I (4).The homology appears to be more conservative toward the amino and carboxyl termini of the molecule, whereas the pentapeptide segment of residues 24-28 in CRF, located toward the center of the chain, seems to be dissimilar. Analysis of the helical potential of CRF by the method of Chou and Fasman (5) indicated that two large sections of the molecule have very pronounced a-helical potential. When the amino acid sequence of the two regions of high helical potential is projected axially using the "Edmundson wheel" (6), the hydrophilic and hydrophobic residues appear to be segregated on opposite sides of the cylindrical helix, as shown in Fig. 1. The hydrophilic and hydrophobic domains on these a-helices are not of equal importance: the hydrophobic domain is almost twice as large as the hydrophilic one. The occurrence of regions of high amphiphilic a-helical potential in CRF is not unique to this hormone: in fact, several peptide hormones of intermediate size (i....
