The thylakoid membrane of the chloroplast is the center of oxygenic photosynthesis. To better understand the function of the luminal compartment within the thylakoid network, we have carried out a systematic characterization of the luminal thylakoid proteins from the model organism Arabidopsis thaliana. Our data show that the thylakoid lumen has its own specific proteome, of which 36 proteins were identified. Besides a large group of peptidyl-prolyl cis-trans isomerases and proteases, a family of novel PsbP domain proteins was found. An analysis of the luminal signal peptides showed that 19 of 36 luminal precursors were marked by a twin-arginine motif for import via the Tat pathway. To compare the model organism Arabidopsis with another typical higher plant, we investigated the proteome from the thylakoid lumen of spinach and found that the luminal proteins from both plants corresponded well. As a complement to our experimental investigation, we made a theoretical prediction of the luminal proteins from the whole Arabidopsis genome and estimated that the thylakoid lumen of the chloroplast contains ϳ80 proteins.The ability to perform oxygenic photosynthesis belongs to the distinguishing characteristics of higher plants, algae, and cyanobacteria. In higher plants, the center of the photosynthetic process is the thylakoid membrane of the chloroplast. Here, in a synergistic series of reactions, four protein complexes, the photosystems I and II, the cytochrome b 6 f complex, and the ATP-synthase, produce NADPH and ATP that fuel the further synthesis of carbohydrates (1, 2).A key feature in the energy conversion of photosynthesis is the link between the electron transfer from photosystem II to I via the cytochrome b 6 f complex and the generation of a proton gradient over the thylakoid membrane. To balance the flow of electrical charges during the formation of the proton gradient, there is a busy traffic of chloride and calcium ions from the stroma into the lumen and of magnesium ions from the lumen into the stroma (3-6). This ion traffic plays a fundamental role for the proper function of photosynthesis. For a long time it was believed that accumulating protons and balancing the ion currents over the thylakoid membrane was the main function of the luminal compartment. The ensemble of known luminal proteins was small and consisted of the three extrinsic photosystem II proteins (PsbO, PsbP, and PsbQ) and plastocyanin. This group was later joined by some new proteins such as violaxanthin de-epoxidase (7), polyphenol oxidase (8, 9), the extrinsic photosystem I protein PsaN (10), and the carboxylterminal processing protease for the D1 protein (11).To achieve a more profound understanding of content and functions of the thylakoid lumen, we designed a method that enabled us to isolate a highly pure fraction of luminal proteins from spinach thylakoids. For the first time, we showed that the lumen of the thylakoid membrane contained at least 20 proteins and that the protein concentration of this compartment was similar to tha...

). † The first two authors contributed equally to this paper, and this work is the result of an equal collaboration between the laboratories of the last two authors. SummaryCold acclimation and over-wintering by herbaceous plants are energetically expensive and are dependent on functional plastid metabolism. To understand how the stroma and the lumen proteomes adapt to low temperatures, we have taken a proteomic approach (difference gel electrophoresis) to identify proteins that changed in abundance in Arabidopsis chloroplasts during cold shock (1 day), and short-(10 days) and longterm (40 days) acclimation to 5°C. We show that cold shock (1 day) results in minimal change in the plastid proteomes, while short-term (10 days) acclimation results in major changes in the stromal but few changes in the lumen proteome. Long-term acclimation (40 days) results in modulation of the proteomes of both compartments, with new proteins appearing in the lumen and further modulations in protein abundance occurring in the stroma. We identify 43 differentially displayed proteins that participate in photosynthesis, other plastid metabolic functions, hormone biosynthesis and stress sensing and signal transduction. These findings not only provide new insights into the cold response and acclimation of Arabidopsis, but also demonstrate the importance of studying changes in protein abundance within the relevant cellular compartment.

While research on T cell exhaustion in context of cancer particularly focuses on CD8+ cytotoxic T cells, the role of inhibitory receptors on CD4+ T-helper cells have remained largely unexplored. TIGIT is a recently identified inhibitory receptor on T cells and natural killer (NK) cells. In this study, we examined TIGIT expression on T cell subsets from CLL patients. While we did not observe any differences in TIGIT expression in CD8+ T cells of healthy controls and CLL cells, we found an enrichment of TIGIT+ T cells in the CD4+ T cell compartment in CLL. Intriguingly, CLL patients with an advanced disease stage displayed elevated numbers of CD4+ TIGIT+ T cells compared to low risk patients. Autologous CLL-T cell co-culture assays revealed that depleting CD4+ TIGIT+ expressing T cells from co-cultures significantly decreased CLL viability. Accordingly, a supportive effect of TIGIT+CD4+ T cells on CLL cells in vitro could be recapitulated by blocking the interaction of TIGIT with its ligands using TIGIT-Fc molecules, which also impeded the T cell specific production of CLL-prosurvival cytokines. Our data reveal that TIGIT+CD4+T cells provide a supportive microenvironment for CLL cells, representing a potential therapeutic target for CLL treatment.