Yeongho Kim

Core chlorophytes possess glycerol-3-phosphate dehydrogenases (GPDs) with an unusual bidomain structure, consisting of a glycerol-3-phosphate phosphatase (GPP) domain fused to canonical GPD domains. These plastid-localized enzymes have been implicated in stress responses, being required for the synthesis of glycerol under high salinity and triacylglycerols under nutrient deprivation. However, their regulation under varying environmental conditions is poorly understood. C. reinhardtii transgenic strains expressing constitutively bidomain GPD2 did not accumulate glycerol or triacylglycerols in the absence of any environmental stress. Although the glycerol contents of both wild type and transgenic strains increased significantly upon exposure to high salinity, cycloheximide, an inhibitor of cytoplasmic protein synthesis, abolished this response in the wild type. In contrast, GPD2 transgenic strains were still capable of glycerol accumulation when cultured in medium containing cycloheximide and NaCl. Thus, the pre-existing GPD2 protein appears to become activated for glycerol synthesis upon salt stress. Interestingly, staurosporine, a non-specific inhibitor of protein kinases, prevented this post-translational GPD2 protein activation. Structural modeling analyses suggested that substantial conformational rearrangements, possibly triggered by high salinity, may characterize an active GPD2 GPP domain. Understanding this mechanism(s) may provide insights into the rapid acclimation responses of microalgae to osmotic/salinity stress.

The trans‐Golgi Network (TGN) sorts molecular “addresses” and sends newly synthesized proteins to their destination via vesicular transport carriers. Despite the functional significance of packaging processes at the TGN, the sorting of soluble proteins remains poorly understood. Recent research has shown that the Golgi resident protein Cab45 is a significant regulator of secretory cargo sorting at the TGN. Cab45 oligomerizes upon transient Ca 2+ influx, recruits soluble cargo molecules (clients), and packs them in sphingomyelin‐rich transport carriers. However, the identity of client molecules packed into Cab45 vesicles is scarce. Therefore, we used a precise and highly efficient secretome analysis technology called hiSPECs. Intriguingly, we observed that Cab45 deficient cells manifest hypersecretion of lysosomal hydrolases. Specifically, Cab45 deficient cells secrete the unprocessed precursors of prosaposin (PSAP) and progranulin (PGRN). In addition, lysosomes in these cells show an aberrant perinuclear accumulation suggesting a new role of Cab45 in lysosomal positioning. This work uncovers a yet unknown function of Cab45 in regulating lysosomal function.