Towards solving the mystery of peroxisomal matrix protein import.

Peroxisomes are vital metabolic organelles that import their lumenal (matrix) enzymes from the cytosol using mobile receptors. Surprisingly, the receptors can even import folded proteins, but the underlying mechanism has been a mystery. Recent results reveal how import receptors shuttle cargo into peroxisomes.

Protein import into peroxisomes occurs through a nuclear pore-like phase.

Peroxisomes are ubiquitous organelles whose dysfunction causes fatal human diseases. Most peroxisomal proteins are imported from the cytosol in a folded state by the soluble receptor PEX5. How folded cargo crosses the membrane is unknown.

Structure and function of the peroxisomal ubiquitin ligase complex.

Peroxisomes are membrane-bounded organelles that exist in most eukaryotic cells and are involved in the oxidation of fatty acids and the destruction of reactive oxygen species. Depending on the organism, they house additional metabolic reactions that range from glycolysis in parasitic protozoa to the production of ether lipids in animals and antibiotics in fungi. The importance of peroxisomes for human health is revealed by various disorders - notably the Zellweger spectrum - that are caused by defects in peroxisome biogenesis and are often fatal.

A peroxisomal ubiquitin ligase complex forms a retrotranslocation channel.

Peroxisomes are ubiquitous organelles that house various metabolic reactions and are essential for human health. Luminal peroxisomal proteins are imported from the cytosol by mobile receptors, which then recycle back to the cytosol by a poorly understood process. Recycling requires receptor modification by a membrane-embedded ubiquitin ligase complex comprising three RING finger domain-containing proteins (Pex2, Pex10 and Pex12).

Liquid-Liquid Phase Transition Drives Intra-chloroplast Cargo Sorting.

In eukaryotic cells, organelle biogenesis is pivotal for cellular function and cell survival. Chloroplasts are unique organelles with a complex internal membrane network. The mechanisms of the migration of imported nuclear-encoded chloroplast proteins across the crowded stroma to thylakoid membranes are less understood.

Chloroplast retrograde signal regulates flowering.

Light is a major environmental factor regulating flowering time, thus ensuring reproductive success of higher plants. In contrast to our detailed understanding of light quality and photoperiod mechanisms involved, the molecular basis underlying high light-promoted flowering remains elusive. Here we show that, in Arabidopsis, a chloroplast-derived signal is critical for high light-regulated flowering mediated by the FLOWERING LOCUS C (FLC).

Plastid-nucleus communication involves calcium-modulated MAPK signalling.

Chloroplast retrograde signals play important roles in coordinating the plastid and nuclear gene expression and are critical for proper chloroplast biogenesis and for maintaining optimal chloroplast functions in response to environmental changes in plants. Until now, the signals and the mechanisms for retrograde signalling remain poorly understood. Here we identify factors that allow the nucleus to perceive stress conditions in the chloroplast and to respond accordingly by inducing or repressing specific nuclear genes encoding plastid proteins.

DEG9, a serine protease, modulates cytokinin and light signaling by regulating the level of ARABIDOPSIS RESPONSE REGULATOR 4.

Cytokinin is an essential phytohormone that controls various biological processes in plants. A number of response regulators are known to be important for cytokinin signal transduction. ARABIDOPSIS RESPONSE REGULATOR 4 (ARR4) mediates the cross-talk between light and cytokinin signaling through modulation of the activity of phytochrome B.

Convergence of light and chloroplast signals for de-etiolation through ABI4-HY5 and COP1.

Seedling de-etiolation prepares plants to switch from heterotrophic to photoautotrophic growth, a transition essential for plant survival. This delicate de-etiolation process is precisely controlled by environmental and endogenous signals. Although intracellular plastid-derived retrograde signalling is essential for the de-etiolation process, the molecular nature of these retrograde signals remains elusive(1-3).

Metabolites and chloroplast retrograde signaling.

Intracellular signaling from chloroplast to nucleus followed by a subsequent response in the chloroplast is called retrograde signaling. It not only coordinates the expression of nuclear and chloroplast genes, which is essential for chloroplast biogenesis, but also maintains chloroplast function at optimal levels in response to fluxes in metabolites and changes in environmental conditions. In recent years several putative retrograde signals have been identified and signaling pathways have been proposed.

A chloroplast envelope-bound PHD transcription factor mediates chloroplast signals to the nucleus.

Chloroplast development, maintenance and function depend on the coordinated expression of chloroplast and nuclear genes. The retrograde chloroplast signals are essential in coordinating nuclear gene expression. Although the sources of signals in chloroplasts have been identified and the associated transcription factors in the nucleus extensively studied, the molecular mechanism that relays chloroplast signals to the nucleus remains a mystery.