Identification and adipocyte differentiation-dependent expression of the unique disialic acid residue in an adipose tissue-specific glycoprotein, adipo Q

Recently, we have shown that alpha2,8-linked disialic acid (diSia) residue occurs in glycoproteins more frequently than ever recognized (Sato, C., Fuk uoka, H., Ohta, K., Matsuda, T., Koshino, R., Kobayashi K., Troy, F. A., II , and Kitajima, K. (2000) J. BioL Chem. 275,15422-15431). In the course of identification of the diSia-containing glycoproteins in mammals, the 30-kDa glycoprotein was found in bovine serum. The 30-kDa glycoprotein was shown to be the bovine adipo Q, an adipocyte-specific protein, based on the parti al amino acid sequences and the immuno-cross-reactivity with the recombinan t mouse adipo Q. The bovine adipo, Q was shown to have no N-linked but O-li nked glycan(s) containing the diSia epitope, Neu5Ac alpha2-8Neu5Ac alpha2 - -> 3Gal. Furthermore, the diSia epitope was also found in the mouse adipo Q in serum as well as in the 3T3-L1 cells that are fully differentiated into adipocytes. Notably, among the known alpha2,8-sialyltransferases, only the alpha2,8-sialyltransferase III mRNA was detected in the 3T3-L1 cells at an y stages of differentiation, and the recombinant alpha2,8-sialyltransferase III could sialylate the purified bovine adipo Q. Thus, this study clearly provides the new findings that adipo Q is the diSia-containing glycoprotein and a physiological substrate of alpha2,8-sialyltransferase III, whose sub strates have not been identified so far.