Tissue-specific expression of beta-galactoside alpha-2,6-sialyltransferase. Transcript heterogeneity predicts a divergent polypeptide

Abstract

The beta-galactoside alpha-2,6-sialyltransferase represents a member of a family of sialyltransferases which catalyze the terminal addition of sialic acid to maturing carbohydrate chains. We surveyed rat tissues using cDNA probes complementary to coding and noncoding domains of the rat liver alpha-2,6-sialyltransferase. In addition to the expected differences in the level of sialyltransferase mRNA among the tissues, there were dramatic qualitative differences as well. Hepatic sialyltransferase probes hybridize to mRNAs of varying size on Northern blots. A tissue-dependent pattern of expression of these transcripts is documented. Evidence is presented that the multiple transcripts are generated from a common gene sequence. At least one instance of alternate splicing in the generation of the kidney sialyltransferase transcripts is predicted by S1 nuclease analysis. We report the isolation of a rat kidney cDNA clone, RKA, that substantiates this tissue-specific alternate splicing event. The RKA insert, although less than full-length, apparently encodes a polypeptide divergent from the reported hepatic alpha-2,6-sialyltransferase (1). RNA blot analysis indicates that the RKA-type transcripts represent a significant proportion of sialyltransferase RNA in rat kidney. Another class of kidney cDNA clones, RKE, is colinear with the hepatic sialyltransferase sequence. RNA blots probed for the divergent and common regions suggest that complex processing pathways are operative in the tissue-specific expression of sialyltransferase mRNA.