Infertility affects 1 in 20 Australia men of reproductive age, and for the majority, the causes remain unknown. In an effort to identify addition key pathways and proteins involved in male fertility we undertook a random mouse mutagenesis screen. In the process we identified the Mot1 mouse line which carries a point mutation in the Rabl2 gene. Mot1 homozygous males are sterile, have a reduced daily sperm output, short sperm tails and drastically reduced progressive sperm motility.

 RABL2 is an uncharacterised member of the RAS GTPase superfamily. Some small GTPases are capable of switching between an inactive and active state and of binding to a specific set of effector proteins that they transport around the cell.  Our data shows that RABL2 is expressed in male haploid germ cells and other ciliated tissues. It interacts with a key pathway involved in flagella development; specifically RABL2 binds to intra-flagella transport (IFT) complex B components. Further, GTP-bound RABL2 binds to a specific set of effector proteins, including key components in glycolytic pathway that it delivers into the growing sperm tail.

 In addition, molecular modeling shows the Mot1 mutation is located within a β sheet required for protein-protein interactions in other GTPases. This model predicts that the Mot1 mutation delay the conversion of RABL2 from the inactive GDP-bound state into the active GTP-bound state. Ultimately this leads to decreased RABL2-effector protein binding and a failure of effector proteins delivery into the tail and sterility. The above results demonstrate the function of previously uncharacterised protein, RABL2, as an essential regulator of sperm flagellum formation and raise the possibility that mutations in RABL2 may underlie cases of human infertility.