N-terminal iron-mediated self-cleavage of human frataxin: regulation of iron binding and complex formation with target proteins

Frataxin is an iron-binding mitochondrial matrix protein that has been shown to mediate iron delivery during iron–sulfur cluster and heme biosynthesis. Mitochondrial processing peptidase (MPP) yields a form of human frataxin corresponding to residues 56–210. However, structural and functional studies have focused on a core structure that results from an ill-defined cleavage event at the N-terminus. Herein we show that the N-terminus of MPP-processed frataxin shows a unique high-affinity iron site and that this iron center appears to mediate a self-cleavage reaction. Moreover, the N-terminus appears to block previously defined iron-binding sites located on the carboxylate-rich surface defined by the helix (α1) and the β-sheet (β1), most likely through electrostatic contact with the carboxylate-rich surface on the core protein, as well as inhibiting iron-promoted binding of the iron–sulfur cluster assembly scaffold partner protein, ISU. The physiological significance of iron-mediated release of the N-terminal residues from this anionic surface is discussed.     

Changes in timing of seasonal peak photosynthetic activity in northern ecosystems

Seasonality in photosynthetic activity is a critical component of seasonal carbon, water, and energy cycles in the Earth system. This characteristic is a consequence of plant's adaptive evolutionary processes to a given set of environmental conditions. Changing climate in northern lands (>30°N) alters the state of climatic constraints on plant growth, and therefore, changes in the seasonality and carbon accumulation are anticipated. However, how photosynthetic seasonality evolved to its current state, and what role climatic constraints and their variability played in this process and ultimately in carbon cycle is still poorly understood due to its complexity. Here, we take the “laws of minimum” as a basis and introduce a new framework where the timing (day of year) of peak photosynthetic activity (DOY_Pmax) acts as a proxy for plant's adaptive state to climatic constraints on its growth. Our analyses confirm that spatial variations in DOY_Pmax reflect spatial gradients in climatic constraints as well as seasonal maximum and total productivity. We find a widespread warming‐induced advance in DOY_Pmax (?1.66 ± 0.30 days/decade, p < 0.001) across northern lands, indicating a spatiotemporal dynamism of climatic constraints to plant growth. We show that the observed changes in DOY_Pmax are associated with an increase in total gross primary productivity through enhanced carbon assimilation early in the growing season, which leads to an earlier phase shift in land‐atmosphere carbon fluxes and an increase in their amplitude. Such changes are expected to continue in the future based on our analysis of earth system model projections. Our study provides a simplified, yet realistic framework based on first principles for the complex mechanisms by which various climatic factors constrain plant growth in northern ecosystems.