Transfection analysis of expression of mRNA isoforms encoding the nuclear autoantigen La/SS-B.

Transcription of the gene encoding for the nuclear autoantigen La resulted in La mRNA isoforms. A promoter switching combined with an alternative splicing pathway replaced the exon 1 with the exon 1'. The exon 1' contained GC-rich regions and an oligo(U) tail of 23 uridine residues. Moreover, it encoded for three open reading frames upstream of the La protein reading frame. Despite this unusual structure, when exon 1' La mRNAs were expressed in transfected cells, both exon 1 and 1' La mRNAs were translated to La protein, whereas the upstream open reading frames of the exon 1' were not translated. In addition to full-length exon 1' La mRNAs 5'-shortened exon 1' La mRNAs were detected. The exon 1' 5'-starts varied in dependence on the analyzed tissues. Like the full-length exon 1' La mRNA a 5'-shortened exon 1' construct starting downstream of the oligo(U) tail but upstream of the open reading frames 2 and 3 was also well translated when transfected in mouse cells. Thus all La mRNA forms represent functional La mRNAs.