Blom7 alpha Is a Novel Heterogeneous Nuclear Ribonucleoprotein K Homology Domain Protein Involved in Pre-mRNA Splicing That Interacts with SNEVPrp19-Pso4

The removal of introns from pre-mRNA is performed by the spliceosome that stepwise assembles on the pre-mRNA before performing two catalytic steps. The spliceosome-associated CDC5L-SNEVPrp19-Pso4 complex is implicated in activation of the second catalytic step of pre-mRNA splicing, and one of its members, SNEVPrp19-Pso4, is also implicated in spliceosome assembly. To identify interaction partners of SNEVPrp19-Pso4, we have performed yeast two-hybrid screenings. Among the putative binding partners was a so far uncharacterized protein carrying two heterogeneous nuclear ribonucleoprotein K homology domains that we termed Blom7 alpha. Blom7 alpha is expressed in all tissues tested, and at least three splice variants exist. After confirming direct and physical interaction of SNEV and Blom7 alpha, we investigated if it plays a functional role during pre-mRNA splicing. Indeed, Blom7 alpha co-localizes and co-precipitates with splicing factors and pre-mRNA and is present in affinity-purified spliceosomes. More importantly, addition of Blom7 alpha to HeLa nuclear extracts increased splicing activity in a dose-dependent manner. Furthermore, we tested if Blom7 alpha influences splice site selection using two different minigene constructs. Indeed, both 5'- as well as 3'-site selection was altered upon Blom7 alpha overexpression. Thus we suggest that Blom7 alpha is a novel splicing factor of the K homology domain family that might be implicated in alternative splicing by helping to position the CDC5L-SNEVPrp19-Pso4 complex at the splice sites.