Protein synthesis and secretion are essential to cellular life. While secretory activities may vary in different cell types, what determine the maximum secretory capacity is inherently difficult to study. Increasing protein synthesis until reaching the limit of secretory capacity is one strategy to address this key issue. Under highly optimized growth conditions, recombinant CHO cells engineered to produce a model human IgG clone started housing rod-shaped crystals in the ER lumen. The intra-ER crystal growth was accompanied by cell en-largement and multi-nucleation, and continued until crystals outgrew cell size to breach mem-brane integrity. The intra-ER crystals were composed of correctly folded, endoglycosidase H-sensitive IgG. Crystallizing propensity was due to the intrinsic physicochemical properties of the model IgG and the crystallization was reproduced in vitro by exposing a high concentration IgG to a near neutral pH. The striking cellular phenotype implicated the efficiency of IgG protein synthesis and oxidative folding exceeded the capacity of ER export machinery. As a result, export-ready IgG accumulated progressively in the ER lumen until a threshold concentration was reached to nucleate crystals. Using an in vivo system that reports accumulation of correctly-folded IgG, we showed the ER-to-Golgi transport steps became rate limiting in cells with high secretory activity.
The article is published online The Journal of Biological Chemistry and is free to access.