Pore-forming protein βγ-CAT drives extracellular nutrient scavenging under cell starvation

Abstract

Nutrient acquisition is essential for cells. βγ-CAT is a pore-forming protein (PFP) and trefoil factor complex assembled under tight regulation identified in toad Bombina maxima. Here, we reported that B. maxima cells secreted βγ-CAT under glucose and glutamine deficiency to scavenge extracellular proteins for their nutrient supply and survival. AMP-activated kinase signaling positively regulated the expression and secretion of βγ-CAT. The PFP complex promoted albumin and ovalbumin uptake through endolysosomal pathways. Elevated intracellular amino acids, enhanced ATP production, and eventually prolonged cell survival were observed in the presence of βγ-CAT and extracellular albumin or ovalbumin. Liposome assays indicated that high concentration of ATP (around 1–5 mM) negatively regulated the opening of βγ-CAT channels. Collectively, these results uncovered that βγ-CAT is an essential element in cell nutrient scavenging under cell starvation by driving vesicular uptake of extracellular proteins, providing a new paradigm for PFPs in cell nutrient acquisition and metabolic flexibility.

Caenorhabditis elegansLIN‐24, a homolog of bacterial pore‐forming toxin, protects the host from microbial infection

“…In addition to playing an important anti-microbial role, our recent studies also showed that the B. maxima af-PFP complex βγ-CAT can promote tissue repair, 44 help toads maintain water balance under osmotic stress, 45 and acquire or deliver nutrients under starvation conditions. 14,15 These findings suggested that af-PFPs may play multiple roles in toads depending on environmental conditions. Similarly, transcriptomic analysis of LIN-24 RNAi and LIN-24-OE worms showed that LIN-24 is involved in pathways other than those associated with the immune response, such as longevity regulation, fatty acid degradation, membrane transport, oxidoreductase regulation, and metabolism (Figures 2 and 6).…”

“…Ep37 has been suggested to be related to newt development, 4 and we recently found that βγ-CAT helps protect animals and cells against metabolic stresses such as starvation. 14,15 Overexpression of a flower-specific af-PFP from the dioecious plant Rumex acetosa alters flower development and induces male sterility in transgenic tobacco. 16 However, due to the existence of numerous isoforms within the same species and lack of specific tools such as antibodies or probes, 11 the physiological roles of af-PFPs are difficult to systematically investigate in these species.…”

IgG Fc-binding protein positively regulates the assembly of pore-forming protein complex βγ-CAT evolved to drive cell vesicular delivery and transport

“…Cell membranes with their selective permeability form barriers for molecule exchange between the cytosol and the extracellular environment ( 37 , 38 ). Toad B. maxima PFP complex βγ-CAT is a secretory endolysosome channel protein that has evolved to facilitate cell nutrient and water acquisition and transcellular transport while fulfilling immune defense (( 24 , 25 , 26 , 27 , 31 )). Considering the primary functions of βγ-CAT in B. maxima , there must be endogenous elements in the toad to regulate assembly and dissociation of this PFP complex, which guarantee its specific and controllable biological actions.…”

“…BmTFF3 functions in this PFP complex by acting as a chaperon that stabilizes the BmALP1 monomer and delivers BmALP1 to proper membrane targets via a double-receptor binding model ( 23 , 32 ). Toad B. maxima βγ-CAT is natural example that a TFF can form a PFP complex machine working in extracellular molecule uptake and transport between a cell and the extracellular environment (( 14 , 24 , 25 , 26 , 27 , 31 )). Considering the high conservation and functions of TFFs in vertebrates ( 46 , 48 , 50 , 61 , 62 , 63 , 64 ), our results provide new information on their first-hand actions and mechanisms involved in participating in novel PFP functions.…”

“…In starved toad cells, βγ–CAT drives the production of protein nutrient-containing vesicles, called nutrisomes. These specific vesicles fuse with lysosomes to hydrolyze imported proteins and the resulting amino acid products are released to cytosol to support the nutrient supply and energy fuel in starved cells ( 31 ). Alternatively, specific intracellular vesicles induced by βγ–CAT, which contain imported extracellular substances, such as water with ions ( 26 ) and albumin-bound fatty acids ( 24 ), are exported out of cells by exosome release, resulting in intercellular delivery of the cargo molecules for water acquisition and tissue parenchymal cell nutrient supply, respectively.…”