Unconjugated
DDX3 is a DEAD-box RNA helicase involved in human immunodeficiency virus mRNA export and translation. Previously, we reported that DDX3 is required for cyclin A expression. To examine whether DDX3 is regulated at the post-transcriptional level, we determined the phosphorylation sites of hamster DDX3 in vitro. Threonine 204 (Thr204) is a conserved amino acid residue of DDX3 homologues in yeast, frog, hamster, and human that is located within motif Q of DEAD-box RNA helicases. A Thr204 to Glu204 DDX3 mutant protein lost its function, suggesting that phosphorylation at Thr204 affects DDX3 function. Thr204 was phosphorylated by cyclin B/cdc2. Thr323 in motif Ib was also phosphorylated by cyclin B/cdc2 kinase. We propose a novel function of cyclin B/cdc2 kinase in mitosis, which is to cause a loss of DDX3 function to repress cyclin A expression and to decrease ribosome biogenesis and translation during mitosis.
We investigated the function of DDX3Y, the Y chromosome AZFa region encoding a putative DEAD-box RNA helicase protein, the loss of which results in oligozoospermia or azoospermia in humans. The human DDX3Y amino acid sequence is similar to that of the X chromosome gene DDX3X (91.7% homology). Here we report that human Y- and X-encoded DEAD box RNA helicase proteins DDX3Y and DDX3X are interchangeable and have an essential function: both proteins rescued a temperature-sensitive mutant hamster cell line (tsET24) that was otherwise incapable of growth at a nonpermissive temperature. Mouse homologues Ddx3y and D1Pas1-PL10 also rescued the mutant cell line at a nonpermissive temperature. In situ hybridization revealed that Ddx3x mRNA was expressed in almost every cell in mouse testis, suggesting that Ddx3x is involved in spermatogenesis. A comparative study of DDX3X and DDX3Y was performed to determine the significance of DDX3Y for cell growth and spermatogenesis. Both DDX3X and DDX3Y promoter DNAs produced a similar degree of transcription in vivo, whereas deletion studies of the promoter DNAs indicated that these genes are differentially regulated. DDX3Y, similar to DDX3X, shuttles between the nucleus and cytoplasm in a crm1-dependent manner.
