Two novel effectors of trafficking and maturation of the yeast plasma membrane H+ -ATPase.

The endoplasmic reticulum (ER) is the entry site of proteins into the endomembrane system. Proteins exit the ER via coat protein II (COPII) vesicles in a selective manner, mediated either by direct interaction with the COPII coat or aided by cargo receptors. Despite the fundamental role of such receptors in protein sorting, only a few have been identified. To further define the machinery that packages secretory cargo and targets proteins from the ER to Golgi membranes, we used multiple systematic approaches, which revealed 2 uncharacterized proteins that mediate the trafficking and maturation of Pma1, the essential yeast plasma membrane proton ATPase. Ydl121c (Exp1) is an ER protein that binds Pma1, is packaged into COPII vesicles, and whose deletion causes ER retention of Pma1. Ykl077w (Psg1) physically interacts with Exp1 and can be found in the Golgi and coat protein I (COPI) vesicles but does not directly bind Pma1. Loss of Psg1 causes enhanced degradation of Pma1 in the vacuole. Our findings suggest that Exp1 is a Pma1 cargo receptor and that Psg1 aids Pma1 maturation in the Golgi or affects its retrieval. More generally our work shows the utility of high content screens in the identification of novel trafficking components.

The back and forth of cargo exit from the endoplasmic reticulum.

Endoplasmic reticulum (ER) to Golgi trafficking is an essential step in sorting mature, correctly folded, processed and assembled proteins (cargo) from immature proteins and ER-resident proteins. However, the mechanisms governing trafficking selectivity, specificity and regulation are not yet fully understood. To date, three complementary mechanisms have been described that enable regulation of this trafficking step: ER retention of immature proteins in the ER; selective uptake of fully mature proteins into Golgi-bound vesicles; and retrieval from the Golgi of immature cargo that has erroneously exited the ER. Together, these three mechanisms allow incredible specificity and enable the cell to carry out protein quality control and regulate protein processing, oligomerization and expression. This review will focus on the current knowledge of selectivity mechanisms acting during the ER-to-Golgi sorting step and their significance in health and disease. The review will also highlight several key questions that have remained unanswered and discuss the future frontiers.

From organelle to organ: ZRIZI MATE-Type transporter is an organelle transporter that enhances organ initiation.

Plant architecture is a predictable but flexible trait. The timing and position of organ initiation from the shoot apical meristem (SAM) contribute to the final plant form. While much progress has been made recently in understanding how the site of leaf initiation is determined, the mechanism underlying the temporal interval between leaf primordia is still largely unknown. The Arabidopsis ZRIZI (ZRZ) gene belongs to a large gene family encoding multidrug and toxic compound extrusion (MATE) transporters. Unique among plant MATE transporters identified so far, ZRZ is localized to the membrane of a small organelle, possibly the mitochondria. Plants overexpressing ZRZ in initiating leaves are short, produce leaves much faster than wild-type plants and show enhanced growth of axillary buds. These results suggest that ZRZ is involved in communicating a leaf-borne signal that determines the rate of organ initiation.