ANDROFEST 2014* Protein transport and sperm tail growth (#7)
In an effort to define the processes of sperm tail formation we have undertaken a random mutagenesis screen to identify mouse lines containing male infertility-causing mutations that result in asthenospermia. Several lines were identified including one that contained a mutation in the Rabl2 gene (Mot mouse line) and another containing a mutation in the Lrguk gene (Kaos mouse line).
RABL2 is a member of the small GTPase family that is expressed in many tissues, but highly enriched in elongating spermatids. The Rabl2Mot mutation affects cycling of RABL2 between the GTP-bound active state and the GDP-bound inactive state. RABL2 dysfunction caused by either the Rabl2Mot mutation, or in a complete knockout, leads to the formation of relatively normal looking sperm but with shortened tails with a severely compromised capacity for progressive motility. Based on this phenotype and the function of analogous GTPases we hypothesized that the presence of the Rabl2Mot mutation would lead to a failure to deliver cargo proteins into the growing sperm tails. In support of this hypothesis, functional analyses identified several RABL2 effector proteins including components of the fibrous sheath that in the present of mutant RABL2 were transported into sperm tails in greatly reduced quantities. Additional binding data indicates that RABL2 interfaces with the intra-flagellar transport system.
By contrast LRGUK is an extremely poorly characterized protein. The LrgukKaos mutation results in the truncation and subsequent loss specifically of LRGUK isoform 1, and leads to male sterility characterized by a virtual absence of sperm tail development, and the presence of fragmented and poorly attached acrosomes. Yeast two-hybrid and immunoprecipitation data revealed that LRGUK-1 binds to the adaptor protein HOOK2 and that they co-localize in a common protein transport tract characterized by the progressive localization to the acrosome, acroplaxome, manchette and ultimately the sperm tails. A closer examination of LrgukKaos/Kaossperm revealed the presence of a persistent failure of basal body (centriole) attachment to the plasma membrane and abnormal sub-distal appendages. The later was consistent with the failure of axoneme extension from the basal body.
