RAB-Like 2 (RABL2) function in male fertility, intra-flagella transport and sperm tail assembly (#54)
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 line which carries a point mutation in the Rabl2 gene. Mot1 homozygous male 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. Many RAS proteins are involved in membrane trafficking and cell signaling. 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 highly in male haploid germ cells and other ciliated tissues and interacts with a key pathway involved in flagella development. Specifically RABL2 binds to intra-flagella transport (IFT) complex B particles. 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.
Further, molecular modeling shows the Mot1 mutation is located within a beta sheet required for protein-protein interactions. We predict the Mot1 mutation leads to a delayed conversion of RABL2 from the inactive GDP-bound state into the active GTP-bound state. We have shown 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 function and raise the possibility that mutations in Rabl2 may underlie cases of human infertility.