Crystal structure of human ZAG, a fat-depleting factor related to MHC molecules

Zn-alpha(2)-glycoprotein (ZAC) is a soluble protein that is present in seru m and other body fluids. ZAG stimulates lipid degradation in adipocytes and causes the extensive fat tosses associated with some advanced cancers. The 2.8 angstrom crystal structure of ZAC resembles a class I major histocompa tibility complex (MHC) heavy chain, but ZAG does not bind the class I light chain beta(2)-microglobutin. The ZAG structure includes a large groove ana logous to class I MHC peptide binding grooves. Instead of a peptide, the ZA G groove contains a nonpeptidic compound that may be implicated in lipid ca tabolism under normal or pathological conditions.