Negative regulator of the copia retrotransposon element of the white (w) gene. In the eye, it is required for normal pigmentation, photoreceptor cell development and for organization of interommatidial bristles. Also essential for embryonic segmentation and differentiation of the nervous system. Functions in the control of alternative splicing by phosphorylating the arginine/serine-rich splicing factors, SR proteins. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] ATP + L-tyrosyl-[protein] = ADP + H(+) + O-phospho-L-tyrosyl-[protein] Mainly nuclear with only low levels present in the cytoplasm. Additional isoforms seem to exist. Ubiquitous expression in embryos. Stage 17 embryos show elevated expression in CNS and brain. Ubiquitous expression in larval imaginal disks. Increased expression posterior to the eye-antennal disk morphogenetic furrow. Expressed both maternally (isoform C) and zygotically (isoforms A and C) in all developmental stages. Autophosphorylated on serine, threonine and tyrosine residues. May correspond to the described 105 kDa isoform although there is no translation sequence evidence for this. Belongs to the protein kinase superfamily. CMGC Ser/Thr protein kinase family. Lammer subfamily. Chimera. Chimera of genomic DNA and cDNA.