Computing the Relative Affinity of Chlorophylls a and b to Light-Harvesting Complex II.

In plants and algae, the primary antenna protein bound to photosystem II is light-harvesting complex II (LHCII), a pigment-protein complex that binds eight chlorophyll (Chl) a molecules and six Chl b molecules. Chl a and Chl b differ only in that Chl a has a methyl group (-CH3) on one of its pyrrole rings, while Chl b has a formyl group (-CHO) at that position. This blue-shifts the Chl b absorbance relative to Chl a. It is not known how the protein selectively binds the right Chl type at each site. Knowing the selection criteria would allow the design of light-harvesting complexes that bind different Chl types, modifying an organism to utilize the light of different wavelengths. The difference in the binding affinity of Chl a and Chl b in pea and spinach LHCII was calculated using multiconformation continuum electrostatics and free energy perturbation. Both methods have identified some Chl sites where the bound Chl type (a or b) has a significantly higher affinity, especially when the protein provides a hydrogen bond for the Chl b formyl group. However, the Chl a sites often have little calculated preference for one Chl type, so they are predicted to bind a mixture of Chl a and b. The electron density of the spinach LHCII was reanalyzed, which, however, confirmed that there is negligible Chl b in the Chl a-binding sites. It is suggested that the protein chooses the correct Chl type during folding, segregating the preferred Chl to the correct binding site.

BMP pathway regulation of insulin signaling components promotes lipid storage in Caenorhabditis elegans.

A small number of peptide growth factor ligands are used repeatedly in development and homeostasis to drive programs of cell differentiation and function. Cells and tissues must integrate inputs from these diverse signals correctly, while failure to do so leads to pathology, reduced fitness, or death. Previous work using the nematode C. elegans identified an interaction between the bone morphogenetic protein (BMP) and insulin/IGF-1-like signaling (IIS) pathways in the regulation of lipid homeostasis. The molecular components required for this interaction, however, were not fully understood. Here we report that INS-4, one of 40 insulin-like peptides (ILPs), is regulated by BMP signaling to modulate fat accumulation. Furthermore, we find that the IIS transcription factor DAF-16/FoxO, but not SKN-1/Nrf, acts downstream of BMP signaling in lipid homeostasis. Interestingly, BMP activity alters sensitivity of these two transcription factors to IIS-promoted cytoplasmic retention in opposite ways. Finally, we probe the extent of BMP and IIS interactions by testing additional IIS functions including dauer formation, aging, and autophagy induction. Coupled with our previous work and that of other groups, we conclude that BMP and IIS pathways have at least three modes of interaction: independent, epistatic, and antagonistic. The molecular interactions we identify provide new insight into mechanisms of signaling crosstalk and potential therapeutic targets for IIS-related pathologies such as diabetes and metabolic syndrome.

Caenorhabditis elegans DBL-1/BMP Regulates Lipid Accumulation via Interaction with Insulin Signaling.

Metabolic homeostasis is coordinately controlled by diverse inputs. Understanding these regulatory networks is vital to combating metabolic disorders. The nematode Caenorhabditis elegans has emerged as a powerful, genetically tractable model system for the discovery of lipid regulatory mechanisms. Here we introduce DBL-1, the C. elegans homolog of bone morphogenetic protein 2/4 (BMP2/4), as a significant regulator of lipid homeostasis. We used neutral lipid staining and a lipid droplet marker to demonstrate that both increases and decreases in DBL-1/BMP signaling result in reduced lipid stores and lipid droplet count. We find that lipid droplet size, however, correlates positively with the level of DBL-1/BMP signaling. Regulation of lipid accumulation in the intestine occurs through non-cell-autonomous signaling, since expression of SMA-3, a Smad signal transducer, in the epidermis (hypodermis) is sufficient to rescue the loss of lipid accumulation. Finally, genetic evidence indicates that DBL-1/BMP functions upstream of Insulin/IGF-1 Signaling in lipid metabolism. We conclude that BMP signaling regulates lipid metabolism in C. elegans through interorgan signaling to the Insulin pathway, shedding light on a less well-studied regulatory mechanism for metabolic homeostasis.