HUMAN RSK ISOFORMS - CLONING AND CHARACTERIZATION OF TISSUE-SPECIFIC EXPRESSION

Serine-threonine protein kinases in the ribosomal S6 kinase (rsk or p9 0(rsk)) family have been implicated as signaling intermediates in the cellular response to several growth factors. To investigate the molecu lar diversity of human p90(rsk) isoforms, mixed degenerate oligonucleo tide polymerase chain reaction was used to isolate partial rsk cDNAs ( 1.1 kb). Three closely related human rsk cDNAs were obtained (HU-1, HU -2, HU-3). These cDNAs are encoded by separate genes based on DNA sequ ence diversity and distinct patterns seen with genomic Southern blots. Northern analysis revealed different sized mRNA transcripts for each isoform. A full-length HU-1 cDNA (3.1 kb) was subsequently isolated fr om a HeLa cell library. 5'-cDNA clones for HU-2 and HU-3 were isolated using the ''rapid amplification of cDNA ends'' strategy. Experiments using human x hamster somatic cell hybrids localized the HU-1 gene to human chromosome 3; HU-2 is on chromosome 6; and HU-3 is on the X chro mosome. The tissue distribution of human rsk mRNAs was determined usin g ribonuclease protection assays. HU-3 mRNA was present in multiple RN A samples. HU-2 was expressed in fibroblast > muscle > lymphocyte = pl acenta > liver. HU-1 was expressed in Epstein-Barr virus lymphocyte > > muscle = liver > fat = placenta. These results indicate that the mul tiplicity of p90(rsk) isoforms is increased to at least three for huma ns and that marked tissue-/cell-specific differences in p90(rsk) isofo rm expression are present.