Identification of the growth hormone-releasing peptide binding site in CD36: a photoaffinity cross-linking study

The GHRPs (growth hormone-releasing peptides) are a class of small synthetic peptides known to stimulate GH release through binding of a G-protein-coupled receptor (designated GHS-R). We have found that hexarelin, a hexapeptide member of the GHRPs, binds to another protein identified as CD36, a scavenger receptor that is expressed in various tissues, including monocytes/macrophages and the endothelial microvasculature. CD36 is involved in the endocytosis of oxLDL (oxidized low-density lipoprotein) by macrophages, and in the modulation of angiogenesis elicited by thrombospondin-1 through binding to endothelial cells. To define the binding domain for hexarelin on CD36, covalent photolabelling of CD36 followed by enzymic and chemical degradation of the photoligand–receptor complex was performed. A 8 kDa photolabelled fragment corresponding to the CD36-(Asn¹³²–Glu¹⁷⁷) sequence has been identified as the hexarelin-binding site. Chemical cleavage of this fragment with CNBr resulted in the release of the free ligand, suggesting that Met¹⁶⁹ is the contact point for the ligand within the receptor binding pocket. We conclude that the binding domain for hexarelin on CD36 overlaps with that for oxLDL, which corresponds to residues Gln¹⁵⁵–Lys¹⁸³ of CD36. Hence hexarelin might interfere with the CD36-mediated uptake of modified lipoproteins by macrophages. This may contribute, at least in part, to the anti-atherosclerotic effect of GHRPs in apolipoprotein E-deficient mice.