Retrograde flux, or the net movement of cargo from the plus end to the soma, was therefore examined by fluorescently labeling lysosomes in DRG neurons and endosomes in the fly. By photobleaching a region close to the neurite tip and then watching see more the transit of those cargoes through the bleached region, both groups observed that, in the absence of CAP-Gly domain, these organelles were not leaving the endings in appropriate numbers, although

they were correctly delivered to the distal tips. Promoting the initiation of retrograde transport represents a new neuronal function for p150s CAP-Gly domain. If this is the main function of the CAP-Gly domain and anterograde transport is unaffected, one would expect distal accumulations of dynein and its cargo. In fact, Lloyd et al. (2012) noticed gross accumulations of endosome components, neuronal membranes, and dynein in the distal boutons of fly neurons when the CAP-Gly domain was lacking. Moughamian and Holzbaur (2012) also looked for such accumulations in DRG neurons but did not see them. This phenotypic distinction is a curious difference between the studies but may not reflect a species difference in the function of the domain so much as the conditions studied. The fly neuromuscular junction has differentiated terminal boutons

in which the cargo piles up, but the ongoing axonal growth in DRG cultures may have allowed dynein and its cargoes to be dispersed as the neurite extended. Significant differences may and nonetheless exist in the manner in which cells handle the initiation of retrograde HCS assay transport. In mammalian neurons, although p150 is enriched at plus ends, little dynein accumulates. p150 at the plus ends may capture and rapidly tether arriving dynein for the immediate initiation of cargo loading and retrograde transport. However, in fungi, not just p150,

but all of the dynactin/dynein complex and LIS1 are enriched at hyphal plus end tips through an interaction of the EB1-like fungal protein, Peb1, and p150s CAP-Gly domain. In those cells, dynactin and dynein are delivered but are not released for retrograde transport until triggered by the separate delivery of early endosomes (Lenz et al., 2006). Thus, some cell types may elect only to keep dynactin on hand at the plus end (through the EB1/EB3/p150 interaction; Figure 1B), while other cells store dynein there as well. The mechanism regulating initiation of motor activity will likely differ between cell types. Both the Perry syndrome and HMN7B mutations occur within the CAP-Gly domain of p150 (Figure 1A), and both are autosomal-dominant diseases, but whereas HMN7B, like amyotrophic lateral sclerosis, causes degeneration specifically of motor neurons, Perry syndrome most prominently affects the substania nigra and brainstem and causes Parkinsonian symptoms.