Two Pathways for Trafficking the Borate Receptor BOR1

Polar localization of plasma membrane proteins is an important way cells regulate signaling pathways, transport across membranes, and growth. Polarization has obvious cellular effects, but localization of membrane components also contributes significantly to processes such as tip growth in root hairs and pollen tubes, phytohormone signaling, and directional micronutrient transport (Šamaj et al., 2006; Grunewald and Friml, 2010; Leitner et al., 2010; Luschnig and Vert, 2014). For example, in the transport of the essential micronutrient boron, the borate exporter BOR1 localizes to the membranes of pericycle cells, where it helps to load boron into the xylem (Takano et al., 2002). BOR1 levels in the plasma membrane change in response to boron levels, with high levels of boron inducing internalization and degradation of BOR1 in the vacuole (Kasai et al., 2014).

In their recent report, Yoshinari et al. (2019) explore the separate endocytic pathways regulating BOR1 localization to the plasma membrane and degradation in the vacuole. In the trafficking of BOR1, recycling between the plasma membrane and trans-Golgi network/early endosomes maintains the polar localization of BOR1, and a degradation pathway from the plasma membrane to multivesicular bodies/late endosomes helps BOR1 make its way to the vacuole. The main question asked by Yoshinari et al. (2019) is what pathways mediate the polar localization and vacuolar sorting of BOR1? As shown in this report, the two pathways that manage BOR1 membrane localization include one that relies on the clathrin adaptor protein 2 (AP2) for BOR1 polarization at the plasma membrane and a clathrin/AP2-independent pathway that functions in BOR1 degradation.

As observed for the PIN auxin transport proteins, endocytic recycling is a crucial part of maintaining BOR1 polarization (Kleine-Vehn et al., 2011). Several plasma membrane proteins share a YXXΦ motif recognized by AP subunits that sort or target them for vesicle transport (Park et al., 2013; Teh et al., 2013; Wang et al., 2016). In plants, loss-of-function mutants of AP2 subunits have pleiotropic effects at the cellular and physiological levels, most likely because of the misregulation of the plasma membrane proteins that they help transport/localize (Fan et al., 2013; Yamaoka et al., 2013). To separate the endocytic pathways involved in BOR1 polar localization and degradation, Yoshinari et al. (2019) used BOR1-GFP constructs expressed in ap2m mutants and observed a disrupted localization pattern and a reduced level of internalization from the plasma membrane compared with the wild type. However, BOR1 endocytosis initiated by high-boron conditions was not impaired in ap2m mutants, suggesting that the degradation pathway of BOR1 is AP2 independent. Indeed, high boron-induced internalization of BOR1 was only affected by a K590A point mutation, which affects ubiquitination of the C-terminal tail of BOR1. It is interesting that the C-terminal tail seems to be involved in both endocytic polarization and degradation pathways, but not exclusively through AP2 or other clathrin-mediated endocytic machinery. This dual regulation at the C terminus provides new evidence of how endocytic regulator proteins potentially recognize multiple interaction sites within their target membrane proteins.

The BOR1 and PIN proteins include a YXXΦ motif, but interestingly, Yoshinari et al. (2019) show that altering this motif does not affect BOR1-AP2 interactions; rather, these interactions occur through domains residing on the C-terminal tail, suggesting that BOR1 internalization occurs through its interaction with AP2, is unique from other AP subunits, and does not require the conserved YXXΦ motif. It will be interesting to follow the influence this result has on future characterization of how BOR1 protein structure contributes to the dynamics of its interactions with AP proteins and how these multiple interaction domains help specialize BOR1 localization. Moreover, this work points to clathrin- or AP-independent endocytic pathways at the plasma membrane that may help specialize the regulation of transport and signaling molecules during plant growth and environmental responses (Fig. 1). It also identifies where more questions could be asked to improve our descriptions of endocytic specialization and protein recycling versus degradation and how these independent pathways function together to coordinate cell function.

Figure 1.

Separation of AP2-dependent and -independent endocytic trafficking of BOR1. Yoshinari et al. (2019) provide a new model for BOR1 endocytosis that involves AP2 for maintaining polar localization of BOR1 transporters at the plasma membrane (PM; A and B) and an AP2-independent degradation pathway that is potentially driven by alternative endocytic machinery that relies on the ubiquitination of the C-terminal tail of BOR1 (C–E). MVB/LE, Multivesicular bodies/late endosomes; TGN/EE, trans-Golgi network/early endosomes.