Biochemical Characterization of the Sexually Dimorphic Helicases DDX3X and DDX3Y.

DEAD-box helicases play a crucial role in the metabolism of cellular RNAs. One key member of this family is the helicase DDX3X, which is encoded on the X chromosome. Recently, the Y chromosome-encoded homolog of DDX3X, DDX3Y, has been shown to be expressed at the protein level in several tissues, such as in the heart and in nervous tissue. Herein, we provide the first measurements of the catalytic activity of DDX3Y as it compares to DDX3X. Using both continuous and single time point ATPase measurements, we found DDX3Y to be a less active ATPase than DDX3X. Furthermore, using truncated forms of both proteins, we find that the intrinsically disordered regions (IDRs) of both proteins affect their ATPase activities to different degrees. Because the ATPase activity of DEAD-box proteins is known to be RNA-triggered, we also assayed the RNA binding propensity of these proteins. We found that, in agreement with our ATPase data, DDX3Y binds double stranded RNA less tightly than DDX3X. These findings may prove important to human health and disease, given that nearly half the population has a Y chromosome and thus expresses some amount of DDX3Y protein.