Targeting Drosophila Sas6 to mitochondria reveals its high affinity for Gorab.

The ability to relocalise proteins to defined sub-cellular locations presents a powerful tool to examine protein-protein interactions that can overcome a tendency of non-targeted exogenous proteins to form inaccessible aggregates. Here we show that a 24 amino-acid sequence from the Drosophila proapoptotic protein Hid's Tail Anchor (HTA) domain can target exogenous proteins to the mitochondria in Drosophila cells. We use this HTA-tag to target the Drosophila centriole cartwheel protein Sas6 to the mitochondria and show that both exogenous and endogenous Gorab can be co-recruited from the Golgi to the new mitochondrial site. This accords with our previous observation that monomeric Drosophila Gorab binds Sas6 to become centriole associated with a 50-fold greater affinity than dimeric Gorab binds Rab6 to become localised at the Golgi. Strikingly, Drosophila Sas6 can bind both Drosophila Gorab and its human GORAB orthologue whereas human Sas6 is unable to bind either GORAB or Gorab. We discuss these findings in relation to the evolutionary conservation of Gorab and the divergence of Sas6, possibly reflecting known differences in persistence of the cartwheel in the centriole duplication cycle of fly and human cells.