Description: Focal adhesion turnover during cell migration is an integrated cyclic process requiring tight regulation of integrin function. Interaction of integrin with its ligand depends on its activation state, which is regulated by the direct recruitment of proteins onto the β integrin chain cytoplasmic domain. We previously reported that ICAP-1α, a specific cytoplasmic partner of β1A integrins, limits both talin and kindlin interaction with β1 integrin, thereby restraining focal adhesion assembly. Here we provide evidence that the calcium and calmodulin-dependent serine/threonine protein kinase type II (CaMKII) is an important regulator of ICAP-1α for controlling focal adhesion dynamics. CaMKII directly phosphorylates ICAP-1α and disrupts an intramolecular interaction between the N- and the C-terminal domains of ICAP-1α, unmasking the PTB domain, thereby permitting ICAP-1α binding onto the β1 integrin tail. ICAP-1α direct interaction with the β1 integrin tail and the modulation of β1 integrin affinity state are required for down-regulating focal adhesion assembly. Our results point to a molecular mechanism for the phosphorylation-dependent control of ICAP-1α function by CaMKII, allowing the dynamic control of β1 integrin activation and cell adhesion.

Description: Mono- and digalactosyldiacylglycerol (MGDG and DGDG) are the most abundant lipids of photosynthetic membranes (thylakoids). In Arabidopsis green tissues, MGD1 is the main enzyme synthesizing MGDG. This monotopic enzyme is embedded in the inner envelope membrane of chloroplasts. DGDG synthesis occurs in the outer envelope membrane. Although the suborganellar localization of MGD1 has been determined, it is still not known how the lipid/glycolipid composition influences its binding to the membrane. The existence of a topological relationship between MGD1 and "embryonic" thylakoids is also unknown. To investigate MGD1 membrane binding, we used a Langmuir membrane model allowing the tuning of both lipid composition and packing. Surprisingly, MGD1 presents a high affinity to MGDG, its product, which maintains the enzyme bound to the membrane. This positive feedback is consistent with the low level of diacylglycerol, the substrate of MGD1, in chloroplast membranes. By contrast, MGD1 is excluded from membranes highly enriched in, or made of, pure DGDG. DGDG therefore exerts a retrocontrol, which is effective on the overall synthesis of galactolipids. Previously identified activators, phosphatidic acid and phosphatidylglycerol, also play a role on MGD1 membrane binding via electrostatic interactions, compensating the exclusion triggered by DGDG. The opposite effects of MGDG and DGDG suggest a role of these lipids on the localization of MGD1 in specific domains. Consistently, MGDG induces the self-organization of MGD1 into elongated and reticulated nanostructures scaffolding the chloroplast membrane.—Sarkis, J., Rocha, J., Maniti, O., Jouhet, J., Vié, V., Block, M. A., Breton, C., Maréchal, E., Girard-Egrot, A. The influence of lipids on MGD1 membrane binding highlights novel mechanisms for galactolipid biosynthesis regulation in chloroplasts.

Description: Thylakoid membranes, the universal structure where photosynthesis takes place in all oxygenic photosynthetic organisms from cyanobacteria to higher plants, have a unique lipid composition. They contain a high fraction of 2 uncharged glycolipids, the galactoglycerolipids mono- and digalactosyldiacylglycerol (MGDG and DGDG, respectively), and an anionic sulfolipid, sulfoquinovosediacylglycerol (SQDG). A remarkable feature of the evolution from cyanobacteria to higher plants is the conservation of MGDG, DGDG, SQDG, and phosphatidylglycerol (PG), the major phospholipid of thylakoids. Using neutron diffraction on reconstituted thylakoid lipid extracts, we observed that the thylakoid lipid mixture self-organizes as a regular stack of bilayers. This natural lipid mixture was shown to switch from hexagonal II toward lamellar phase on hydration. This transition and the observed phase coexistence are modulated by the fine-tuning of the lipid profile, in particular the MGDG/DGDG ratio, and by the hydration. Our analysis highlights the critical role of DGDG as a contributing component to the membrane stacking via hydrogen bonds between polar heads of adjacent bilayers. DGDG interactions balance the repulsive electrostatic contribution of the charged lipids PG and SQDG and allow the persistence of regularly stacked membranes at high hydration. In developmental contexts or in response to environmental variations, these properties can contribute to the highly dynamic flexibility of plastid structure.—Demé, B., Cataye, C., Block, M. A., Maréchal, E., Jouhet, J. Contribution of galactoglycerolipids to the 3-dimensional architecture of thylakoids.