ISOLATION AND CHARACTERIZATION OF GBP28, A NOVEL GELATIN-BINDING PROTEIN PURIFIED FROM HUMAN PLASMA

By use of its affinity to gelatin-Cellulofine, a novel protein, GBP28 (gelatin-binding protein of 28 kDa), was obtained from human plasma. G BP28 bound to gelatin-Cellulofine could be eluted with 1 M NaCl. By an alysis of its amino-terminal amino acid sequences and the peptides obt ained by protease digestion, GBP28 was identified as a novel protein. After repeated gel chromatography of the 1 M NaCl eluate from gelatin- Cellulofine about 50 mu g of GBP28 was purified from 500 ml of human p lasma. On gel chromatography, the protein migrated as a molecule of ab out 420 kDa. On SDS-PAGE, its molecular mass was 28 kDa under reducing conditions and 68 kDa under nonreducing conditions. Recently, human m RNA specific to adipose tissue, cDNA clone apM1, has been registered [ Maeda, K., Okubo, K., Shimomura, I., Funahashi, T., Matsuzawa, Y., and Matsubara, K. (1996) Biochem. Biophys. Res. Commun. 221, 286-289]. Th e assumed amino acid sequence of cDNA clone apM1 contained all the seq uences of GBP28 and its peptides. Therefore, it is evident that the c- DNA clone apM1 encodes GBP28 and the protein is specific to adipose ti ssue. The clone encodes a polypeptide of 244 amino acids with a secret ory signal sequence at the amino terminus, a small non-helical region, a stretch of 22 collagen repeats and a globular domain. Thus, GBP28 a ppears to belong to a family of proteins possessing a collagen-like do main through which they form homo-trimers, which further combine to ma ke oligomeric complexes. Although its biological function is presently unclear, its adipocyte-specific expression suggests that GBP28 may fu nction as an endogenous factor involved in lipid catabolism and storag e or whole body metabolism.