POTENTIAL FOR 2 ISOFORMS OF THE A1-RIBONUCLEOPROTEIN IN XENOPUS-LAEVIS

We have identified the Xenopus cognates for the RNA-binding protein A1. This protein has previously been shown to be one of the components of the heterogeneous nuclear ribonucleoprotein (hnRNP) complex in rat and human cells. We have isolated several Xenopus clones from oocyte, tailbud embryo, and leg muscle cDNA libraries and determined their nucleotide sequences. Potentially, two different A1 isoforms are expressed in Xenopus; they have been termed XA1a and XA1b. Besides insertions and deletions, the XA1a protein sequence is 92% identical to the rat protein and suggests very similar secondary structures. There are two segments in the COOH-terminal domain where deletions or insertions are apparent: the rat protein does not have a 48-residue sequence that is present in the frog protein, and the frog protein does not have a 12-residue sequence that is present in the rat protein. We have confirmed that the XA1a protein is larger than rat A1 by in vitro transcription, translation, and gel electrophoresis. The second isoform, XA1b, is very similar to the XA1a isoform, except it has a different COOH terminus due to the absence of a 73-nucleotide region from its cDNA clones. Transcripts representing both isoforms have been detected in various Xenopus RNA preparations by polymerase chain reaction experiments with A1-specific oligonucleotides. Our findings suggest that the isoforms are encoded by one or two genes and are the result of alternative splicing. We discuss the biological implications of having two forms of the Al component of hnRNP particles.