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The coiled-coil domains of intermediate filament (IF) proteins are flanked by regions of low sequence complexity. Whereas IF coiled-coil domains assume dimeric and tetrameric conformations on their own, maturation of eight tetramers into cylindrical IFs is dependent upon either “head” or “tail” domains of low sequence complexity. Here we confirm that the tail domain required for assembly ofDrosophilaTm1 IFs functions by forming labile cross-β interactions. These interactions are seen in polymers made from the tail domain alone as well as assembled IFs formed by the intact Tm1 protein. The ability to visualize such interactionsin situwithin the context of a discrete cellular assembly lends support to the concept that equivalent interactions may be used in organizing other dynamic aspects of cell morphology.One Sentence SummaryA new form of protein folding that interconverts between the structured and unstructured states controls assembly of intermediate filaments.

Localization and local translation ofoskarmRNA at the posterior pole of theDrosophilaoocyte directs abdominal patterning and germline formation in the embryo. The process requires recruitment and precise regulation of motor proteins to form transport-competent mRNPs. We show that the posterior-targeting kinesin-1 is loaded upon nuclear export ofoskarmRNPs, prior to their dynein-dependent transport from the nurse cells into the oocyte. We demonstrate that kinesin-1 recruitment requires theDmTropomyosin1-I/C isoform, an atypical RNA-binding tropomyosin that binds directly to dimerizingoskar3’UTRs. Finally, we show that a small but dynamically changing subset ofoskarmRNPs gets loaded with inactive kinesin-1 and that the motor is activated during mid-oogenesis by the functionalized splicedoskarRNA localization element. This inefficient, dynamic recruitment of Khc decoupled from cargo-dependent motor activation constitutes an optmized, coordinated mechanism of mRNP transport, by minimizing interference with other cargo-transport processes and between the cargo associated dynein and kinesin-1.