Cell type-specific Rab32 and Rab38 cooperate with the ubiquitous lysosome biogenesis machinery to synthesize specialized lysosome-related organelles.

Lysosome-related organelles (LROs) exist in specialized cells to serve specific functions and typically co-exist with conventional lysosomes. The biogenesis of LROs is known to utilize much of the common protein machinery used in the transport of integral membrane proteins to lysosomes. Consequently, an outstanding question in the field has been how specific cargoes are trafficked to LROs instead of lysosomes, particularly in cells that simultaneously produce both organelles. One LRO, the melanosome, is responsible for the production of the pigment melanin and has long been used as a model system to study the formation of specialized LROs. Importantly, melanocytes, where melanosomes are synthesized, are a cell type that also produces lysosomes and must therefore segregate traffic to each organelle. Two small GTPases, Rab32 and Rab38, are key proteins in the biogenesis of melanosomes and were recently shown to redirect the ubiquitous machinery-BLOC-2, AP-1 and AP-3-to traffic specialized cargoes to melanosomes in melanocytes. In addition, the study revealed Rab32 and Rab38 have both redundant and unique roles in the trafficking of melanin-producing enzymes and overall melanosome biogenesis. Here we review these findings, integrate them with previous knowledge on melanosome biogenesis and discuss their implications for biogenesis of other LROs.

AGAP1/AP-3-dependent endocytic recycling of M5 muscarinic receptors promotes dopamine release.

Of the five mammalian muscarinic acetylcholine (ACh) receptors, M(5) is the only subtype expressed in midbrain dopaminergic neurons, where it functions to potentiate dopamine release. We have identified a direct physical interaction between M(5) and the AP-3 adaptor complex regulator AGAP1. This interaction was specific with regard to muscarinic receptor (MR) and AGAP subtypes, and mediated the binding of AP-3 to M(5). Interaction with AGAP1 and activity of AP-3 were required for the endocytic recycling of M(5) in neurons, the lack of which resulted in the downregulation of cell surface receptor density after sustained receptor stimulation. The elimination of AP-3 or abrogation of AGAP1-M(5) interaction in vivo decreased the magnitude of presynaptic M(5)-mediated dopamine release potentiation in the striatum. Our study argues for the presence of a previously unknown receptor-recycling pathway that may underlie mechanisms of G-protein-coupled receptor (GPCR) homeostasis. These results also suggest a novel therapeutic target for the treatment of dopaminergic dysfunction.

Beta-NAP, a cerebellar degeneration antigen, is a neuron-specific vesicle coat protein.

We have identified a target antigen in autoimmune cerebellar degeneration, beta-NAP, that is closely related to the beta-adaptin and beta-COP coat proteins. Beta-NAP is a nonclathrin-associated phosphoprotein expressed exclusively in neurons, from E12 through adulthood. Beta-NAP is present in the neuronal soma and nerve terminal as soluble and membrane-bound pools and is associated with a discrete set of nerve-terminal vesicles. These results establish beta-NAP as a neuron-specific vesicle coat protein. We propose a model in which beta-NAP mediates vesicle transport between the soma and the axon terminus and suggest that beta-NAP may represent a general class of coat proteins that mediates apical transport in polarized cells.