Mitochondria are brought from the cell body to distant axons by kinesin. On page 545, Glater et al. identify two proteins that offer a regulatable link between motor and organelle.
The same group had already shown that the accumulation of mitochondria in photoreceptor axons required a cytoplasmic protein called milton. They now show why milton is so important—it recruits kinesin heavy chain to the mitochondria.
Milton hooks onto a mitochondrial protein called miro, as shown by immunoprecipitation assays and expression of miro mutants. Both miro and milton might offer opportunities for the cell to tune microtubule motility. One alternatively spliced version of milton, for instance, includes the kinesin-binding domain yet does not recruit the motor to mitochondria. Perhaps it is folded into an inert form that blocks transport. This form might even be opened by various cellular signals.
Miro contains calcium-binding and GTPase-like domains. Calcium is abundant at active synapses and requires large amounts of energy to be expelled. The authors therefore speculate that high calcium levels inhibit the transport complex so that mitochondria pause where they are needed.
Kinesin light chain was not found in the transport complex. Although this domain is usually the motor's cargo adaptor, milton seems to substitute for that job here. In theory, then, kinesin could have many adaptors specific to various cargo.