Analysis of kidney mRNAs expressed from the rat beta-galactoside alpha 2,6-sialyltransferase gene

beta-Galactoside alpha 2,6-sialyltransferase (SiaT-1), like other glycosyltransferases, is differentially expressed in rat tissues. Two distinct size classes of SiaT-1 mRNAs expressed in rat kidney are comprised of at least three SiaT-1 transcripts. One mRNA, RKE, represents the larger transcript class (4.7 kb) and predicts a polypeptide identical to the hepatic SiaT-1. In transfected Chinese hamster ovary (CHO) cells, RKE polypeptides exhibit hemi-perinuclear staining with a SiaT-1 antibody (Ab-267) that is consistent with Golgi localization. RKE transfectants display cell-surface alpha 2,6-sialic acid linkages as determined by lectin affinity staining. Two other mRNAs, RKA and RKB, are members of a smaller size class (3.6 kb) that comprise predominant SiaT-1 transcripts in rat kidney. Both RKA and RKB encode polypeptides that are missing the amino-terminal 232 residues, but retain 171 amino acids of RKE carboxy-terminal sequence information. A short, leucine-rich peptide present in the divergent amino-terminus of RKA has sequence similarity to the secretory signal domain of several eukaryotic secretory and cell-surface proteins. In transfected CHO cells, both RKA and RKB polypeptides display an immunostaining pattern that is distinct from that of the Golgi-associated SiaT-1 protein (RKE). Furthermore, RKA or RKB transfectants do not display alpha 2,6-sialic acid linkages on cell-surface glycoconjugates. Consistent with the expression of divergent SiaT-1 mRNAs in rat kidney, protein blot analysis of rat tissue homogenates with Ab-267 reveals that in addition to protein that co-migrates with hepatic SiaT-1, rat kidney expresses a unique size class of SiaT-1 proteins.