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. GBP28 bound to gelatin-Cellulofine could be eluted with 1 M NaCl. By analysis of its amino-terminal amino acid sequences and the peptides obtained 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 micrograms of GBP28 was purified from 500 ml of human plasma. On gel chromatography, the protein migrated as a molecule of about 420 kDa. On SDS-PAGE, its molecular mass was 28 kDa under reducing conditions and 68 kDa under nonreducing conditions. Recently, human mRNA 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]. The assumed amino acid sequence of cDNA clone apM1 contained all the sequences of GBP28 and its peptides. Therefore, it is evident that the cDNA clone apM1 encodes GBP28 and the protein is specific to adipose tissue. The clone encodes a polypeptide of 244 amino acids with a secretory signal sequence at the amino terminus, a small non-helical region, a stretch of 22 collagen repeats and a globular domain. Thus, GBP28 appears to belong to a family of proteins possessing a collagen-like domain through which they form homo-trimers, which further combine to make oligomeric complexes. Although its biological function is presently unclear, its adipocyte-specific expression suggests that GBP28 may function as an endogenous factor involved in lipid catabolism and storage or whole body metabolism.