The major seed storage proteins of maize (Zea mays) and bean (Phaseolus vulgaris), zein and phaseolin, accumulate in the endoplasmic reticulum (ER) and in storage vacuoles, respectively. We show here that a chimeric protein composed of phaseolin and 89 amino acids of γ-zein, including the repeated and the Pro-rich domains, maintains the main characteristics of wild-type γ-zein: It is insoluble unless its disulfide bonds are reduced and forms ER-located protein bodies. Unlike wild-type phaseolin, the protein, which we called zeolin, accumulates to very high amounts in leaves of transgenic tobacco (Nicotiana tabacum). A relevant proportion of the ER chaperone BiP is associated with zeolin protein bodies in an ATP-sensitive fashion. Pulse-chase labeling confirms the high affinity of BiP to insoluble zeolin but indicates that, unlike structurally defective proteins that also extensively interact with BiP, zeolin is highly stable. We conclude that the γ-zein portion is sufficient to induce the formation of protein bodies also when fused to another protein. Because the storage proteins of cereals and legumes nutritionally complement each other, zeolin can be used as a starting point to produce nutritionally balanced and highly stable chimeric storage proteins.

Summary: Ion fluxes are ubiquitous processes in the plant and animal kingdoms, controlled by fine‐tuned regulations of ion channel activity. Yet the mechanism that cells employ to achieve the modification of ion homeostasis at the molecular level still remains unclear. This is especially true when it comes to the mechanisms that lead to cell death. In this study, Arabidopsis thaliana cells were exposed to ozone (O₃). Ion flux variations were analyzed by electrophysiological measurements and their transcriptional regulation by RT‐PCR. Reactive oxygen species (ROS) generation was quantified by luminescence techniques and caspase‐like activities were investigated by laser confocal microscopy. We highlighted the delayed activation of K⁺ outward‐rectifying currents after an O₃‐induced oxidative stress leading to programmed cell death (PCD). Caspase‐like activities are detected under O₃ exposure and could be decreased by K⁺ channel blocker. Molecular experiments revealed that the sustained activation of K⁺ outward current could be the result of an unexpected post‐transcriptional regulation of the guard cell outward‐rectifying K⁺ (GORK) channels. This consists of a likely new mode of regulating the processing of the GORK mRNA, in a ROS‐dependent manner, to allow sustained K⁺ effluxes during PCD. These data provide new mechanistic insights into K⁺ channel regulation during an oxidative stress response.

Recent evidence indicates that ADP-ribosylation factor 1 (ARF1) carries out multiple roles in plant cells that may be independent from the established effector complex COPI. To investigate potential COPI-independent functions, we have followed the dynamics of ARF1 and a novel putative effector, the plant golgin GRIP-related ARF-binding domain-containing Arabidopsis (Arabidopsis thaliana) protein 1 (GDAP1) in living plant cells. We present data that ascribe a new role to ARF1 in plant cell membrane traffic by showing that the GTPase functions to recruit GDAP1 to membranes. In addition, although ARF1 appears to be central to the recruitment of both COPI components and the golgin, we have established a different subcellular distribution of these ARF1 effectors. Live cell imaging demonstrates that GDAP1 and COPI are distributed on Golgi membranes. However, GDAP1 is also found on ARF1-labeled structures that lack coatomer, suggesting that the membrane environment, rather than ARF1 alone, influences the differential recruitment of ARF1 effectors. In support of this hypothesis, fluorescence recovery after photobleaching analyses demonstrated that GDAP1 and COPI have different kinetics on membranes during the cycle of activation and inactivation of ARF1. Therefore, our data support a model where modulation of the cellular functions of ARF1 in plant cells encompasses not only the intrinsic activities of the effectors, but also differential recruitment onto membranes that is spatially regulated.

Protein bodies (PB) are stable polymers naturally formed by certain seed storage proteins within the endoplasmic reticulum (ER). The human immunodeficiency virus negative factor (Nef) protein, a potential antigen for the development of an anti-viral vaccine, is highly unstable when introduced into the plant secretory pathway, probably because of folding defects in the ER environment. The aim of this study was to promote the formation of Nef-containing PB in tobacco (Nicotiana tabacum) leaves by fusing the Nef sequence to the N-terminal domains of the maize storage protein γ-zein or to the chimeric protein zeolin (which efficiently forms PB and is composed of the vacuolar storage protein phaseolin fused to the N-terminal domains of γ-zein). Protein blots and pulse–chase indicate that fusions between Nef and the same γ-zein domains present in zeolin are degraded by ER quality control. Consistently, a mutated zeolin, in which wild-type phaseolin was substituted with a defective version known to be degraded by ER quality control, is unstable in plant cells. Fusion of Nef to the entire zeolin sequence instead allows the formation of PB detectable by electron microscopy and subcellular fractionation, leading to zeolin–Nef accumulation higher than 1% of total soluble protein, consistently reproduced in independent transgenic plants. It is concluded that zeolin, but not its γ-zein portion, has a positive dominant effect over ER quality control degradation. These results provide insights into the requirements for PB formation and avoidance of quality-control degradation, and indicate a strategy for enhancing foreign protein accumulation in plants.