Wei Feng

Among the distinct adaptor protein (AP) complexes, AP-4 primarily functions as a non-clathrin-coated vesicle machinery essential for intracellular membrane trafficking. ARF1 is a master regulator of AP-4 membrane recruitment, but the underlying mechanism remains elusive. Here, we present the cryo-EM structures of soluble AP-4 and the AP-4/ARF1 complex. Unexpectedly, AP-4 adopts a dynamic equilibrium between closed and open conformations, caused by loose contacts between its medium subunit and central core. ARF1 binding induces only subtle changes in AP-4, which retains its conformational equilibrium. Mutations at the AP-4/ARF1 interface disrupt complex formation and impair ARF1-dependent membrane recruitment. Efficient membrane recruitment of AP-4 likely requires the synergistic engagement of ARF1 and cargoes. Disrupting the conformational flexibility of AP-4 interferes with this synergistic effect and compromises AP-4-mediated membrane trafficking. Our findings may redefine AP-4 as a conformationally dynamic complex modulated by cooperative interactions, providing insights into neurodevelopmental disorders associated with AP-4 dysfunction.

Microtubules have been extensively studied for their interactions at the outer surface and ends, yet the regulatory mechanisms within the microtubule lumen, particularly in singlet microtubules, remain poorly understood. Here, we developed a proximity-labeling strategy to distinguish intraluminal from external proteins, enabling the systematic identification of key regulators residing within the singlet microtubule lumen. Among these, we focused on Jupiter microtubule-associated homolog 2 (JPT2), a conserved microtubule-binding protein. JPT2 binds directly to microtubules and localizes to the lumen in a C-terminal-dependent manner. Within this compartment, it modulates the distribution of the α-tubulin acetyltransferase MEC17 and contributes to luminal homeostasis. Notably, the luminal accessibility of JPT2 is highly sensitive to microtubule structural states and is markedly reduced by Paclitaxel treatment, suggesting that drug-induced conformational changes or steric hindrance can restrict luminal entry. Together, our findings identify JPT2 as a key organizer of the singlet microtubule lumen and provide a framework for the spatial and functional analysis of microtubule inner proteins. Beyond exogenous perturbations, our observations suggest that luminal protein distribution may also be modulated by steric competition or selective entry among intraluminal components, indicating a more intricate regulatory landscape within the microtubule interior.

Heterotrimeric kinesin-2, composed of two distinct kinesin motors and a kinesin-associated protein (KAP), is essential for intraflagellar transport and ciliogenesis. KAP specifically recognizes the hetero-paired motor tails for the holoenzyme assembly, but the underlying mechanism remains unclear. Here, we determine the structure of KAP-1 in complex with the hetero-paired tails from kinesin-2 motors KLP-20 and KLP-11. KAP-1 forms an elongated superhelical structure characterized by a central groove and a C-terminal helical (CTH)-hook. The two motor tails fold together and are co-recognized by the central groove of KAP-1. The adjacent hetero-pairing trigger sequences preceding the two tails form an intertwined heterodimer, which co-captures the CTH-hook of KAP-1 to complete the holoenzyme assembly. Mutations in the interfaces between KAP-1 and the two tails disrupt the heterotrimeric kinesin-2 complex and impair kinesin-2-mediated intraflagellar transport. Thus, KAP-1 and the hetero-paired motors are mutually co-recognized, ensuring the proper assembly of heterotrimeric kinesin-2 for cargo transport.

Microtubules are vital components of the cytoskeleton. Their plus ends are dynamic and respond to changes in cell morphology, whereas the minus ends are stable and serve a crucial role in microtubule seeding and maintaining spatial organization. In mammalian cells, the calmodulin-regulated spectrin-associated proteins (CAMSAPs), play a key role in directly regulating the dynamics of non-centrosomal microtubules minus ends. However, the molecular mechanisms are not yet fully understood. Our study reveals that CAMSAP3 forms dimers through its C-terminal α-helix; this dimerization not only enhances the microtubule-binding affinity of the CKK domain but also enables the CKK domain to regulate the dynamics of microtubules. Furthermore, CAMSAP3 also specializes in decorating at the minus end of microtubules through the combined action of the microtubule-binding domain (MBD) and the C-terminal α-helix, thereby achieving dynamic regulation of the minus ends of microtubules. These findings are crucial for advancing our understanding and treatment of diseases associated with non-centrosomal microtubules.

Macropinocytosis mediates the non-selective bulk uptake of extracellular fluid, enabling cells to survey the environment and obtain nutrients. A conserved set of signaling proteins orchestrates the actin dynamics that lead to membrane ruffling and macropinosome formation across various eukaryotic organisms. At the center of this signaling network are Ras GTPases, whose activation potently stimulates macropinocytosis. However, how Ras signaling is initiated and spatiotemporally regulated during macropinocytosis is not well understood. By using the model system Dictyostelium and a proteomics-based approach to identify regulators of macropinocytosis, we uncovered Leep2, consisting of Leep2A and Leep2B, as a RasGAP complex. The Leep2 complex specifically localizes to emerging macropinocytic cups and nascent macropinosomes, where it modulates macropinosome formation by regulating the activities of three Ras family small GTPases. Deletion or overexpression of the complex, as well as disruption or sustained activation of the target Ras GTPases, impairs macropinocytic activity. Our data reveal the critical role of fine-tuning Ras activity in directing macropinosome formation.