Interactive Fly, Drosophila

Molecular motors actively transport many types of cargo along the cytoskeleton in a wide range of organisms. One class of cargo is localized mRNAs, which are transported by myosin on actin filaments or by kinesin and dynein on microtubules. How the cargo is kept at its final intracellular destination and whether the motors are recycled after completion of transport are poorly understood. A new RNA anchoring assay in living Drosophila blastoderm embryos has been used to show that apical anchoring of mRNA after completion of dynein transport does not depend on actin or on continuous active transport by the motor. Instead, apical anchoring of RNA requires microtubules and involves dynein as a static anchor that remains with the cargo at its final destination. This study proposes a general principle that could also apply to other dynein cargo and to some other molecular motors, whereby cargo transport and anchoring reside in the same molecule (Delanoue, 2005).

This study has used a specific RNA anchoring assay to distinguish between the four main models that could explain how apical wg and pair-rule mRNA (runt, and fushi tarazu) are retained in the apical cytoplasm after their transport by dynein. The models that have been proposed could also apply to other molecular motors and their various cargos. (1) The dynein motor could release the RNA cargo at its final destination, allowing the RNA to bind to an actin-dependent static anchor and the motor to participate in further transport. (2) The anchor could be MT associated rather than actin based. (3) RNA could be retained in the apical cytoplasm by continuous active transport without anchoring. (4) The motor itself could retain the cargo and turn into a static anchor when it reaches the final destination (Delanoue, 2005).

At the outset of this study, it was anticipated that cargo anchoring via actin was the most likely possibility given that actin is thought to be involved in anchoring of many other RNAs. It was also thought that after a motor completes a transport cycle, it releases the cargo and is available for transport of new cargo. However, in general, there has not been very good direct evidence showing that such a model is correct because of the lack of an assay that could discriminate between the transport and anchoring steps. In this study, two specific assays were used: one for transport and another for anchoring. Both anchoring and transport were assayed at the same time in the same embryo using two distinct RNAs. These specific assays have allowed a test and refutation of the prevailing actin anchoring model at least in the case of runt, fushi tarazu and wg apical mRNA localization in the Drosophila blastoderm embryo. Against expectations, the results show that the fourth model is correct, namely that wg and pair-rule RNA are anchored by a dynein-dependent mechanism so that the motor molecules are maintained to the site of anchoring with the cargo. The data shows that the requirement for dynein to anchor the apical RNA is independent of the ATPase activity of the motor and its transport cofactors Egl and BicD, all of which are required for the active transport of the RNA. These observations are best explained by a model in which the dynein motor involved in apical transport of RNA does not release the cargo and acts as a static anchor at the final destination (Delanoue, 2005).

It is interesting to consider how a dynamic motor such as dynein could turn into a static anchor after completion of cargo transport. Dynein is a large multicomplex motor that is difficult to work with in vitro. Nevertheless, many of the subunits of dynein are defined and the force-generating protein, Dhc, is thought to contain physically distinct ATPase and MT binding domains. It is therefore easy to imagine how the motor could change to a static anchor by remaining attached to MTs via the MT binding domain and losing its ATPase force-generating capacity. Indeed, ATPase-independent MT binding has been observed with dynein under in vitro conditions. While it is difficult to compare in vitro studies with the current studies in vivo, the latter are likely to show much more complex and varied interactions with proteins in the cell. Indeed, anchoring may also involve interactions with additional components not present in vitro, such as MT-associated proteins (MAPs), which could stabilize the binding of dynein to the apical MTs or could physically obstruct the motor movement. Another possibility could be anchoring through association with ribosomes, but this can be ruled out in the case of wg and pair-rule RNA, since RNAs lacking a coding region can be transported and anchored correctly. Alternative hypotheses, which cannot be ruled out, include a change of conformation or modifications of the structure of the dynein-dynactin complex. While the data demonstrate conclusively a new RNA-anchoring function for dynein, they do not allow distinguishing between the various hypotheses of how this anchoring occurs at the molecular level, nor test definitively whether Dynactin is required for anchoring. p50/dynamitin is present with the anchored RNA, and overexpression of p50/dynamitin and a Glued/p150 allele cause a partial inhibition of RNA localization with no obvious effects on anchoring. These results suggest, but do not demonstrate conclusively, that Dynactin is not required for anchoring. Furthermore, while it is shown that the ATPase activity of the motor is not required for anchoring, this observation does not test whether dynactin is required in addition to dynein for anchoring (Delanoue, 2005).

Whatever the molecular basis for the dynein anchoring function that was uncovered, it seems likely that the described anchoring does not involve a single dynein molecule anchoring a single RNA molecule. Instead, the RNA cargo is likely to consist of particles containing many RNA molecules and probably many motor complexes. The cargo is thus likely to remain strongly attached to at least some motor molecules throughout transport and anchoring. However, it is not yet known what the linkers between the RNA and motors are (Delanoue, 2005).

Little is also known about the mechanism of anchoring of other dynein cargos, although the mechanism of transport of RNA by dynein could be very similar to other cargos such as lipid droplets. Dynein is also required for nuclear positioning and tethering in many systems, so its role as a static anchor may be widespread. Furthermore, some kinesin-like proteins are also thought to interact with static cell components, and recent in vitro studies show that myosin VI can switch from a motor to an anchor under tension. This process has been proposed to stabilize actin cytoskeletal structures and link protein complexes to actin structures. It is therefore proposed that myosins, kinesins, and dynein may all be able to switch under certain circumstances from dynamic motors to static anchors and that the observations of this study may represent a general principle for anchoring of some cargos following transport to their final cytoplasmic destination (Delanoue, 2005).

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