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

Abstract

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.

Mutant Huntingtin Impairs Post-Golgi Trafficking to Lysosomes by Delocalizing Optineurin/Rab8 Complex from the Golgi Apparatus

“…MPR, LcB, and VSV-G expressed in mhtt cells showed a reduced mobile fraction compared with wt cells (see Figure 8C and Supplemental optineurin in the Golgi of mhtt cells. The fact that htt interacts with optineurin (Hattula and Peranen, 2000) and also with the Golgi membranes, suggest that htt may play a central role in the correct localization and function of optineurin in the Golgi apparatus. This is in agreement with our observation that silencing htt by siRNA delocalized optineurin from the Golgi apparatus.…”

“…Interestingly, the colocalization between optineurin and GT-YFP was lower in wt cells with reduced expression of htt with respect to those transfected with control siRNA (Figure 4C), indicating that htt is implicated in the localization of optineurin at the Golgi apparatus. In addition to htt, optineurin also interacts with Rab8 (Hattula and Peranen, 2000), which has been localized to the trans-Golgi network (TGN) (Huber et al, 1993), participating in post-Golgi trafficking (Ang et al, 2003). Thus, we next examined whether mhtt also alters the presence of Rab8 in the Golgi apparatus.…”

“…Htt colocalizes with the Golgi apparatus and with exocytic vesicles (Difiglia et al, 1995) and reduced expression of htt by RNA interference (RNAi) disrupts the Golgi apparatus (Caviston et al, 2007). Moreover, several htt-interacting proteins, such as HIP14 (Singaraja et al, 2002) and optineurin (Hattula and Peranen, 2000), contribute to the function of the Golgi apparatus and exocytosis.…”

“…Optineurin, a Rab8 effector (Hattula and Peranen, 2000), colocalizes with htt in the Golgi apparatus where it contributes to post-Golgi trafficking and Golgi organization (Sahlender et al, 2005). Rab8 belongs to the family of Rab GTPases, central regulators of vesicle budding, motility, and fusion (for review, see Stenmark and Olkkonen, 2001;Pfeffer, 2003).…”

Mutant huntingtin can paradoxically protect neurons from death

“…[29][30][31][32][33] Other candidates that possibly could interact with the domain of the protein encoded by exon 1, but have not been shown to interact only with this short sequence, include huntingtin interacting protein 1 (HIP1), Rab11 familyinteracting protein 2 (FIP2), huntingtin interacting protein 14 (HIP14), nuclear receptor corepressor 1 (N-CoR) and cystathionine beta-synthase (CBS) [34][35][36][37][38] (for overview see Figure 2). …”