Subcellular localization of C-term-oleosin fused to β-casein reveals unexpected cytoplasmic accumulation in vacuole-targeted arabidopsis seeds.

Casein is a high-quality protein source containing all amino acids, which are vital for human nutrition, and play a vital role in granting the texture and mouthfeel of dairy products. Plant-seed production may be an efficient strategy for alternative protein production due to its ability to produce complex proteins, perform post-translational modifications, and be resource-efficient. Targeting recombinant proteins to specific subcellular compartments plays a crucial role in ensuring proper folding, stability, and accumulation. To determine the most suitable subcellular compartment for accumulation of bovine β-Casein fused to plant C-terminal Oleosin (CTO-Cas) different signal peptides were tested, directing the chimeric protein to endoplasmic-reticulum, vacuole or to the chloroplast. These gene constructs were transformed into Arabidopsis using Agrobacterium-mediated transformation by the floral dip technique. CTO-Cas was successfully detected in transformed seeds which were targeted to the vacuole, measured at 1.26% of the Total Soluble Protein (TSP). Subcellular compartmentation of the vacuole-targeted chimeric protein was determined by Transmitting Electron Microscopy (TEM) using gold-immuno-labeling. CTO-Cas was detected in spherical bodies, which are not vacuoles. Major morphological changes were observed in the transgenic seed cells in comparison to WT. Large oil bodies dominated WT seed cells, while seeds expressing CTO-Cas targeted to the vacuole had fewer large oil bodies. Instead, they exhibited numerous small oil bodies alongside dense sub-cellular aggregates only observed in the transgenic seeds. These aggregates were positively gold-immunolabeled with anti-Casein antibodies as well as anti-oil body associated protein antibodies, suggesting the formation of recombinant CTO-Cas aggregates tightly associated with small oil bodies.