p24 family proteins have been known for a long time, but their functions have remained elusive. However, they are emerging as essential regulators of protein trafficking along the secretory pathway, influencing the composition, structure, and function of different organelles in the pathway, especially the ER and the Golgi apparatus. In addition, they appear to modulate the transport of specific cargos, including GPI-anchored proteins, G-protein-coupled receptors, or K/HDEL ligands. As a consequence, they have been shown to play specific roles in signaling, development, insulin secretion, and the pathogenesis of Alzheimer's disease. The search of new putative ligands may open the way to discover new functions for this fascinating family of proteins.

SUMMARYThe p24 proteins belong to a family of type I membrane proteins which cycle between the endoplasmic reticulum (ER) and Golgi via coat protein I (COPI) and COPII vesicles. Current nomenclature classifies them into four subfamilies, although plant p24 proteins belong to either the p24b or the p24d subfamilies. Here, we show that Arabidopsis p24d5/d9 and HDEL ligands shift the steady-state distribution of the K/HDEL receptor ERD2 from the Golgi to the ER. We also show that p24d5/d9 interact directly with ERD2. This interaction requires the Golgi dynamics (GOLD) domain in p24d5 and is much higher at acidic than at neutral pH, consistent with both proteins interacting at the cis-Golgi. In addition, p24d5 also inhibits the secretion of HDEL ligands, but not constitutive secretion, showing a role for p24d5 in retrograde Golgi-to-ER transport. Both p24d5 and ERD2 interact with ADP-ribosylation factor 1 (ARF1) and COPI subunits, mostly at acidic pH, consistent with COPI vesicles being involved in retrograde transport of both proteins. In contrast, both proteins interact with the COPII subunit Sec23, mostly at neutral pH, consistent with this interaction taking place at the ER for anterograde transport to the Golgi apparatus.

The endoplasmic reticulum (ER) is responsible for the synthesis of one third of the cellular proteome and is constantly challenged by physiological and environmental situations that can perturb its homeostasis and lead to the accumulation of misfolded secretory proteins, a condition referred to as ER stress. In response, the ER evokes a set of intracellular signaling processes, collectively known as the unfolded protein response (UPR), which are designed to restore biosynthetic capacity of the ER. As single cell organisms evolved into multicellular life, the UPR complexity has increased to suit their growth and development. In this review, we discuss recent advances in the understanding in the UPR, emphasizing conserved UPR elements between plants and metazoans, and highlighting unique plant-specific features.

HighlightIn Arabidopsis α2-COP is required for plant growth, Golgi structure and subcellular localization of p24δ5, with its loss of function resulting in upregulation of the COPII subunit SEC31A.

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