FIP-2, a coiled-coil protein, links Huntingtin to Rab8 and modulates cellular morphogenesis

Huntington's disease is characterised by the death of cortical and striatal neurons, and is the result of an expanded polyglutamine tract in the Huntingtin protein [1]. Huntingtin is present on both endocytic and secretory membrane organelles but its function is unclear [2,3]. Rab GTPases regulate both of these transport pathways [4]. We have previously shown that Rab8 controls polarised membrane transport by modulating cell morphogenesis [5]. To understand Rab8-mediated processes, we searched for Rab8-interacting proteins by the yeast two-hybrid system. Here, we report that Huntingtin is linked to the Rab8 protein through FIP-2, a tumour necrosis factor-alpha (TNF-alpha)-inducible coiled-coil protein related to the NEMO protein [6,7]. The activated form of Rab8 interacted with the amino-terminal region of FIP-2, whereas dominant-negative Rab8 did not. Huntingtin bound to the carboxy-terminal region of FIP-2. Coexpressed FIP-2 and Huntingtin enhanced the recruitment of Huntingtin to Rab8-positive vesicular structures, and FIP-2 promoted cell polarisation in a similar way to Rab8. We propose a model in which Huntingtin, together with FIP-2 and Rab8, are part of a protein network that regulates membrane trafficking and cellular morphogenesis.