Abstract
SummaryMicrotubule-dependent organization of membranous organelles occurs through motor-based pulling and by coupling microtubule dynamics to membrane remodeling. For example, tubules of endoplasmic reticulum (ER) can be extended by kinesin- and dynein-mediated transport and through the association with the tips of dynamic microtubules. The binding between ER and growing microtubule plus ends requires End Binding (EB) proteins and the transmembrane protein STIM1, which form a tip-attachment complex (TAC), but it is unknown whether these proteins are sufficient for membrane remodeling. Furthermore, EBs and their partners undergo rapid turnover at microtubule ends, and it is unclear how highly transient protein-protein interactions can induce load-bearing processive motion. Here, we reconstituted membrane tubulation in a minimal system with giant unilamellar vesicles, dynamic microtubules, an EB protein, and a membrane-bound protein that can interact with EBs and microtubules. We showed that these components are sufficient to drive membrane remodeling by three mechanisms: membrane tubulation induced by growing microtubule ends, motor-independent membrane sliding along microtubule shafts, and membrane pulling by shrinking microtubules. Experiments and modeling demonstrated that the first two mechanisms can be explained by adhesion-driven biased membrane spreading on microtubules. Optical trapping revealed that growing and shrinking microtubule ends can exert forces of ∼0.5 and ∼5 pN, respectively, through attached proteins. Rapidly exchanging molecules that connect membranes to dynamic microtubules can thus bear a sufficient load to induce membrane deformation and motility. Furthermore, combining TAC components and a membrane-attached kinesin in the same in vitro assays demonstrated that they can cooperate in promoting membrane tubule extension.
Highlights
Microtubules (MTs) are major cytoskeletal filaments, which can generate forces required for many cellular processes
Whereas extraction of membrane tubes by motors moving on stabilized MTs has been extensively characterized by in vitro reconstitution experiments [23,24,25,26,27], the mechanisms underlying tip attachment complex (TAC)-mediated membrane remodeling are unclear
Cell-biological experiments demonstrated that endoplasmic reticulum (ER)-resident STIM1 accumulates at growing MT plus ends in End Binding (EB)-dependent manner, because it contains an EB-binding MT Tip Localization Signal (MtLS) (Figure 1B) [28]
Summary
Microtubules (MTs) are major cytoskeletal filaments, which can generate forces required for many cellular processes. MT-based motors can produce force to position and shape cellular organelles [1]. Dynamic MTs generate pushing and pulling forces in a motor-independent fashion [2, 3]. It has been proposed that the attachment of cellular structures to growing MT ends by MT plus-end-tracking proteins (+TIPs) can lead to force generation. The interaction of EB1 and the transmembrane ER protein STIM1 promotes extension of ER tubules [6]. This mechanism of ER remodeling, driven by the membrane-MT tip attachment complex (TAC) (Figure 1A) [6,7,8,9], represents one of the molecular pathways that shape and distribute ER membranes
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