Different pathways for post-Golgi sorting of proteins to lysosomes/vacuoles have been described for yeast and animals. The pathway through CCVs appears to be conserved among all eukaryotes (plants, yeast, animals). Inside the TGN, specific sorting signals are recognized by TGN membrane localized receptors, recruited into CCVs and transported into LVs/lysosomes [165, 178]. In animal cells, the sorting of acid hydrolases to the lysosome is facilitated by the MPR . MPR-ligand complexes are recruited into CCVs at the TGN. This process is mediated by Golgi-localized, γ adaptin ear-containing, Arf-binding proteins and by the AP1 complex through interactions with MPRs tyrosine (YXXФ) and dileucine (LL) motifs at the cytosolic tail [179, 180]. In yeast, the sorting and delivery to the LV is very similar to the MPR pathway of animal cells [181, 182]. This mechanism is assumed to be used for trafficking to plant LVs as well, although LV proteins interact with VSRs, showing no homology to MPRs . Plant VSRs recognize sequence-specific VSDs (e.g. NPIRL-like consensus motifs) in LV targeted proteins, such as barley aleurain [183, 184]. VSRs then interact with AP1 through a tyrosine-based sorting motif (e.g. YMPL) instead of through a dileucine motif as observed in yeast and animals . CCVs, containing cargo-VSR, then bud from the TGN, and discharge their contents after fusion into the prevacuolar compartment [163, 166]. As a result of the lower pH of the prevacuolar compartment, ligands dissociate from their receptor, and the receptor is subsequently recycled back to the Golgi apparatus [185-187]. Importantly, an MPR-independent and a VPS10-independent lysosome sorting pathway were found in animal cells and yeast, respectively [188, 189]. Recent studies on the sorting of tonoplast transporters in Arabidopsis mesophyll protoplasts suggest a similar route in plants (Fig. 1B) . AP3, but not AP1, appears to fulfill a central role in this pathway. In animals, AP3 can recognize both dileucine and tyrosine motifs, whereas, in yeast, only dileucine signals can be recognized [145, 191]. Recent reports suggest that AP3 subunits are involved in the biogenesis, morphology and function of the prevacuolar compartment and vacuoles in plants [151-153].
Remarkably, and uniquely in plants, LV resident proteins can be transported directly from the ER to the LV by means of ER bodies as intermediate compartments, bypassing the Golgi apparatus, as shown in Fig. 1B . Thus, one of the emerging differences that appears to distinguish plants from other eukaryotes is the plasticity of the ER to form protein-, oil- or rubber-containing subcellular structures best termed ER bodies , which either stably accumulate or are transported to the LV. The ER bodies are in most instances spherical, < 1 μm in diameter, and consist of a dense core of a self-assembling or aggregating protein, oil or rubber, and a membrane of ER origin . However, there is some evidence for nonconventional ER trafficking bypassing the Golgi to the lysosome in animal cells . It was proposed that ER bodies in plants can follow a similar path, bypassing the Golgi and directly fusing with LV, as observed under stress conditions [,196].