ATP, phosphorylation and transcription regulate the mobility of plant splicing factors

Serine-arginine-rich (SR) proteins, a family of spliceosomal proteins, function at multiple steps in the assembly of the spliceosome in non-plant systems. Limited studies with metazoan SR splicing factors (ASF/SF2 and SC35) indicated that their mobility is not dependent on ATP and phosphorylation. In addition, inhibition of transcription slightly increased their mobility. Here, we analyzed the mobility of SR45, a plant-specific SR protein with unique domain organization, and SR1/SRp34, a plant homolog of metazoan ASF/SF2, using fluorescence recovery after photobleaching (FRAP) and fluorescence loss in photobleaching (FLIP). Our results show that, in contrast to metazoan SR splicing factors, the movement of the plant SR proteins is dependent on ATP, phosphorylation and transcription. To understand the underlying mechanism for these observations, we carried out mobility analyses with the domain-deletion mutants of SR45 in ATP-depleted cells and in the presence of inhibitors of transcription or phosphorylation. Our results show that the sensitivity of SR45 to these inhibitors is conferred by an RNArecognition motif (RRM) and the serine-arginine-rich (RS) domain 2. These results provide important insights into the mechanisms of plant SR protein movement and suggest fundamental differences in the regulation of the mobility of plant and animal SR splicing factors.