An alternatively spliced miniexon alters the subcellular fate of the human asialoglycoprotein receptor H2 subunit. Endoplasmic reticulum retention and degradation or cell surface expression

Two types of cDNAs encoding the H2 subunit of the human asialoglycoprotein receptor had been cloned, differing only by the presence (H2a) or absence (H2b) of a segment of 15 base pairs (bp), encoding five amino acids (Glu-Gly-His-Arg-Gly) immediately carboxyl-terminal (exoplasmic) to the single membrane-spanning segment. We have cloned and sequenced this region of the H2 gene and showed that the two H2 forms are alternatively spliced variants differing in the presence of a 15-bp miniexon. Both H2 messenger RNAs were found in HepG2 cells, H2b accounting for about 92% of the H2 mRNAs. When expressed in NIH 3T3 cells without the H1 receptor subunit, the two-variant polypeptides exhibit different subcellular fates. H2a is completely retained in and degraded in the endoplasmic reticulum or a related pre-Golgi compartment. In contrast a substantial amount of H2b is processed by Golgi enzymes and reaches the cell surface. Thus, the sole difference determining the subcellular localization of the two forms if the five-amino acid insert in H2a. When a virion-packaged retroviral vector containing H2a cDNA infected 3T3 cells, 70% of the resulting clones expressed H2b and 30% H2a. Thus the 15-bp H2a miniexon can be spliced out, at least during the retrovirus life cycle.