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The proper functioning of organelles depends on their intracellular localization, mediated by motor protein-dependent transport on cytoskeletal tracks. Rather than directly associating with a motor protein, peroxisomes move by hitchhiking on motile early endosomes in the filamentous fungusAspergillus nidulans. However, the cellular function of peroxisome hitchhiking is unclear. Peroxisome hitchhiking requires the protein PxdA, which is conserved within the fungal subphylum Pezizomycotina, but absent from other fungal clades. Woronin bodies are specialized peroxisomes that are also unique to the Pezizomycotina. In these fungi, multinucleate hyphal segments are separated by incomplete cell walls called septa that possess a central pore enabling cytoplasmic exchange. Upon damage to a hyphal segment, Woronin bodies plug septal pores to prevent wide-spread leakage. Here, we tested if peroxisome hitchhiking is important for Woronin body motility, distribution, and function inA. nidulans. We show that Woronin body proteins are present within all motile peroxisomes and hitchhike on PxdA-labeled early endosomes during bidirectional, long-distance movements. Loss of peroxisome hitchhiking by knocking outpxdAsignificantly affected Woronin body distribution and motility in the cytoplasm, but Woronin body hitchhiking is ultimately dispensable for septal localization and plugging.

In eukaryotic cells, intracellular components are organized by the microtubule motors cytoplasmic dynein-1 (dynein) and kinesins, which are linked to cargos via adaptor proteins. While ∼40 kinesins transport cargo toward the plus end of microtubules, a single dynein moves cargo in the opposite direction. How dynein transports a wide variety of cargos remains an open question. The FTS-Hook-FHIP (“FHF”) cargo adaptor complex links dynein to cargo in mammals and fungi. As human cells have three Hooks and four FHIP proteins, we hypothesized that the combinatorial assembly of different Hook and FHIP proteins could underlie dynein cargo diversity. Using proteomic approaches, we determine the protein ‘interactome’ of each FHIP protein. Live-cell imaging and biochemical approaches show that different FHF complexes associate with distinct motile cargos. These complexes also move with dynein and its cofactor dynactin in single-molecule in vitro reconstitution assays. Complexes composed of FTS, FHIP1B, and Hook1/Hook3 co-localize with Rab5-tagged early endosomes via a direct interaction between FHIP1B and GTP-bound Rab5. In contrast, complexes composed of FTS, FHIP2A and Hook2 colocalize with Rab1A-tagged ER-to-Golgi cargos and FHIP2A is involved in the motility of Rab1A tubules. Our findings suggest that combinatorial assembly of different FTS-Hook-FHIP complexes is one mechanism dynein uses to achieve cargo specificity.

In canonical microtubule-based transport, adaptor proteins link cargos to the molecular motors dynein and kinesin. Recently, an alternative mode of transport known as ‘hitchhiking’ was discovered, in which a cargo achieves motility by hitching a ride on an already-motile cargo, rather than attaching to a motor protein. Hitchhiking has been best-studied in two filamentous fungi, Aspergillus nidulans and Ustilago maydis. In U. maydis, ribonucleoprotein complexes, peroxisomes, lipid droplets, and endoplasmic reticulum all hitchhike on early endosomes. In A. nidulans, peroxisomes hitchhike using a putative molecular linker, PxdA, that associates with early endosomes. However, whether other organelles use PxdA to hitchhike on early endosomes is unclear, as are the molecular mechanisms that regulate hitchhiking in A. nidulans. Here we find that the proper distribution of lipid droplets, mitochondria and autophagosomes do not require PxdA, suggesting that PxdA is a molecular linker specific to peroxisomes. We also identify two new pxdA alleles, including a point mutation (R2044P) that disrupts PxdA’s ability to associate with early endosomes and reduces peroxisome movement. Finally, we identify a novel regulator of peroxisome hitchhiking, the phosphatase DipA. DipA co-localizes with early endosomes and its early endosome-association relies on PxdA.

In contrast to the canonical picture of transport by direct attachment to motor proteins, recent evidence shows that a number of intracellular ‘cargos’ navigate the cytoplasm by hitchhiking on motor-driven ‘carrier’ organelles. We describe a quantitative model of intracellular cargo transport via hitchhiking, examining the efficiency of hitchhiking initiation as a function of geometric and mechanical parameters. We focus specifically on the parameter regime relevant to the hitchhiking motion of peroxisome organelles in fungal hyphae. Our work predicts the dependence of transport initiation rates on the distribution of cytoskeletal tracks and carrier organelles, as well as the number, length and flexibility of the linker proteins that mediate contact between the carrier and the hitchhiking cargo. Furthermore, we demonstrate that attaching organelles to microtubules can result in a substantial enhancement of the hitchhiking initiation rate in tubular geometries such as those found in fungal hyphae. This enhancement is expected to increase the overall transport rate of hitchhiking organelles, and lead to greater efficiency in organelle dispersion. Our results leverage a quantitative physical model to highlight the importance of organelle encounter dynamics in non-canonical intracellular transport.SIGNIFICANCEA variety of cellular components are transported via hitchhiking by attaching to other motile organelles. Defects in the molecular machinery responsible for organelle hitchhiking may be linked with neurodegenerative disorders. To date, no comprehensive physical models of this non-canonical mode of transport have been developed. In particular, the connection between molecular- and organelle-scale properties of hitchhiking components and their effect on cellular-scale transport has remained unclear. Here, we investigate the mechanics of hitchhiking initiation and explore organelle interactions that can modulate the efficiency of this process.