Functional properties of p54, a novel SR protein active in constitutive and alternative splicing

The p54 protein was previously identified by its reactivity with an autoantiserum. We report here that p54 is a new member of the SR family of splicing factors, as judged from its structural, antigenic, and functional characteristics. Consistent with its identification as an SR protein, p54 can function as a constitutive splicing factor in complementing splicing- deficient HeLa cell S100 extract. However, p54 also shows properties distinct from those of other SR family members. p54 can directly interact with the 65- kDa subunit of U2 auxiliary factor (U2AF⁶⁵), a protein associated with the 3' splice site. In addition, p54 interacts with other SR proteins but does not interact with the U1 small nuclear ribonucleoprotein U1-70K or the 35- kDa subunit of U2 auxiliary factor (U2AF³⁵). This protein-protein interaction profile is different from those of prototypical SR proteins SC35 and ASF/SF2, both of which interact with U1-70K and U2AF³⁵ but not with U2AF⁶⁵. p54 promotes the use of the distal 5' splice site in E1A pre-mRNA alternative splicing, while the same site is suppressed by ASF/SFZ and SC35. These findings and the differential tissue distribution of p54 suggest that this novel SR protein may participate in regulation of alternative splicing in a tissue- and substrate-dependent manner.