Alternative
mRNA splicing is a mechanism to regulate
protein isoform expression and is regulated by alternative splicing factors. The alternative splicing factor 45 (SPF45) is overexpressed in
cancer, although few
biological effects of SPF45 are known, and few splicing targets have been identified. We previously showed that Extracellular Regulated
Kinase 2 (ERK2) phosphorylation of SPF45 regulates cell proliferation and adhesion to
fibronectin. In this work, we show that Cdc2-like
kinase 1 (Clk1) phosphorylates SPF45 on eight
serine residues. Clk1 expression enhanced, whereas Clk1 inhibition reduced, SPF45-induced exon 6 exclusion from Fas
mRNA. Mutational analysis of the Clk1 phosphorylation sites on SPF45 showed both positive and negative regulation of splicing, with a net effect of inhibiting SPF45-induced exon 6 exclusion, correlating with reduced Fas
mRNA binding. However, Clk1 enhanced SPF45
protein expression, but not
mRNA expression, whereas inhibition of Clk1 increased SPF45 degradation through a
proteasome-dependent pathway. Overexpression of SPF45 or a phospho-mimetic mutant, but not a phospho-inhibitory mutant, stimulated
ovarian cancer cell migration and invasion, correlating with increased
fibronectin expression, ERK activation and enhanced splicing and phosphorylation of full-length
cortactin. Our results demonstrate for the first time that SPF45 overexpression enhances cell migration and invasion, dependent on biochemical regulation by Clk1.