Gadkin

Abstract

Different types of endosomal vesicles show distinct distribution patterns within cells. While early endosomes can be found throughout the cell, recycling endosomal vesicles and tubules tend to cluster near the microtubule organizing center in the perinuclear region in most cell types. The molecular mechanisms underlying the steady-state distribution and dynamics of various types of endosomal vesicles has long remained enigmatic. However, during the past decade it has become evident that microtubule-based motor proteins of the kinesin family play a pivotal role in the positioning of endosomes. Early endosomes were shown to cluster in the perinuclear area in the absence of KIF16B,¹ KIF3A is required for the steady-state distribution of late endosomes/lysosomes,²,and KIF13A directs M6PR-containing vesicles from the TGN to the plasma membrane³ to name only a few examples. In the case of Tf-containing recycling endosomes antibody-injection experiments implicated kinesin-1, a heteromer comprised of KIF5 heavy and KLC light chains, as a motor for their transport towards the cell periphery.⁴ Indeed, KIF5B knockdown experiments confirmed that kinesin-1 is necessary to maintain the peripheral pool of recycling endosomes.⁵ But how is kinesin-1 linked to endosomal vesicles? Work from our own laboratory has identified the AP-1-binding protein Gadkin as a molecular link between AP-1-mediated traffic and kinesin-1-based transport along microtubules.⁵ This work as well as hypothetical models for kinesin-dependent endosomal membrane traffic will be discussed here.

Figures and Tables

Figure 1 Role of Gadkin and its partners AP-1 and kinesin-1 in trafficking of TGN-derived endosomal vesicles along microtubule tracks. (A and B) HeLa cells expressing Gadkin WT (shown in red) were fixed 24 h post-transfection and analyzed by indirect immunofluorescence microscopy with antibodies against Gadkin and endogenous tubulin (shown in green). Blue, DAPI-stained nuclei. Insets, 4–5× magnified view of boxed areas. Scale bar, 2 µm. (C) HeLa cells were fixed and analyzed by indirect immunofluorescence microscopy with antibodies against endogenous Gadkin (shown in green) and endogenous tubulin (shown in red). Blue, DAPI-stained nuclei. Inset, 4× magnified view of boxed area. Scale bar, 2 µm. (D) Hypothetical model for the role of Gadkin in the regulation of recycling endosomal membrane traffic. See text for further details.