Meiotic spindles viewed from inside of the cell, slightly to the side. Blue, actin; green, myosin; red, microtubules.

Classically, we have learnt that kinesin and dynein move along microtubules, whereas myosin binds to actin. But in more recent times, the need for crosstalk between the two cytoskeletons in many cellular processes has become increasingly apparent, and the finding now that Myo10 may bind to microtubules as well as actin offers yet another mechanism by which this crosstalk can occur.

Weber et al., in a recent issue of Nature (431, 325–329; 2004), first reasoned that Myo10 may be a microtubule–F-actin linker on the basis of its primary structure: in its carboxyl terminus there is a MyTH4 domain, which mediates interactions with microtubules in other systems. This suggested that Myo10 might directly associate not only with actin, but also with microtubules. Indeed, Myo10 co-sedimented with microtubules in Xenopus egg extracts, and colocalized with microtubules independently of F-actin. Furthermore, it colocalized with meiotic spindle microtubules specifically at the interface between the spindle and the cortex. Deletion analysis confirmed that the MyTH4–FERM domain of Myo10 was essential for the interaction with microtubules.

When Weber et al. expressed the Myo10 tail domain, which functions as a dominant negative, they observed displacement of the oocyte nucleus from its characteristic asymmetric localization in the animal hemisphere to the cortex. This phenomenon was not the result of microtubule depolymerization and could be reproduced by the addition of anti-Myo10 antibodies. Thus, Myo10 is required for microtubule-dependent asymmetric anchoring of the oocyte nucleus.

Next, the authors tried to understand the basis of the phenotypes they observed in the oocyte — rotation failure, abnormal spindle structure and multiple microtubule organizing centres (MTOCs). These effects are all similar to those that occur after actin depolymerization during meiotic maturation; however, no actin disassembly was observed. Instead, there was a concentration of F-actin in aggregates on or near to spindles or abortive MTOCs, indicating that Myo10 is essential for proper F-actin–meiotic-spindle interactions. Together, these data suggest that this actin-based motor functions to link the microtubule and F-actin cytoskeletons.

As Weber et al. highlight, Myo10 could function in nuclear anchoring by binding to phosphoinositides through its PH domain and by binding to actin and microtubules; alternatively, the motor activity of Myo10 could function in the transport of microtubule-associated spindle components. In either case, this highlights a previously unappreciated function for a myosin in microtubule-based spindle function. Furthermore, as similar MyTH4–FERM domains are present in other myosins, this may be a more general function of other unconventional myosins.