Arabidopsis thaliana myosin XIK is recruited to the Golgi through interaction with a MyoB receptor

Lipid droplets in Arabidopsis thaliana leaves contain myosin-binding proteins and enzymes associated with furan-containing fatty acid biosynthesis

Lipid droplets (LDs) are lipid storage organelles in plant leaves and seeds. Seed LD proteins are well known, and their functions in lipid metabolism have been characterized; however, many leaf LD proteins remain to be identified. We therefore isolated LDs from leaves of the leaf LD-overaccumulating mutant high sterol ester 1 (hise1) of Arabidopsis thaliana by centrifugation or co-immunoprecipitation. We then performed LD proteomics by mass spectrometry and identified 3,206 candidate leaf LD proteins. In this study, we selected 31 candidate proteins for transient expression assays using a construct encoding the candidate protein fused with green fluorescent protein (GFP). Fluorescence microscopy showed that MYOSIN BINDING PROTEIN14 (MYOB14) and two uncharacterized proteins localized to LDs labeled with the LD marker. Subcellular localization analysis of MYOB family members revealed that MYOB1, MYOB2, MYOB3, and MYOB5 localized to LDs. LDs moved along actin filaments together with the endoplasmic reticulum. Co-immunoprecipitation of myosin XIK with MYOB2-GFP or MYOB14-GFP suggested that LD-localized MYOBs are involved in association with the myosin XIK-LDs. The two uncharacterized proteins were highly similar to enzymes for furan fatty acid biosynthesis in the photosynthetic bacterium Cereibacter sphaeroides, suggesting a relationship between LDs and furan fatty acid biosynthesis. Our findings thus reveal potential molecular functions of LDs and provide a valuable resource for further studies of the leaf LD proteome.

Visualizing the dynamics of plant energy organelles

Plant organelles predominantly rely on the actin cytoskeleton and the myosin motors for long-distance trafficking, while using microtubules and the kinesin motors mostly for short-range movement. The distribution and motility of organelles in the plant cell are fundamentally important to robust plant growth and defense. Chloroplasts, mitochondria, and peroxisomes are essential organelles in plants that function independently and coordinately during energy metabolism and other key metabolic processes. In response to developmental and environmental stimuli, these energy organelles modulate their metabolism, morphology, abundance, distribution and motility in the cell to meet the need of the plant. Consistent with their metabolic links in processes like photorespiration and fatty acid mobilization is the frequently observed inter-organellar physical interaction, sometimes through organelle membranous protrusions. The development of various organelle-specific fluorescent protein tags has allowed the simultaneous visualization of organelle movement in living plant cells by confocal microscopy. These energy organelles display an array of morphology and movement patterns and redistribute within the cell in response to changes such as varying light conditions, temperature fluctuations, ROS-inducible treatments, and during pollen tube development and immune response, independently or in association with one another. Although there are more reports on the mechanism of chloroplast movement than that of peroxisomes and mitochondria, our knowledge of how and why these three energy organelles move and distribute in the plant cell is still scarce at the functional and mechanistic level. It is critical to identify factors that control organelle motility coupled with plant growth, development, and stress response.

Keep in contact: multiple roles of endoplasmic reticulum-membrane contact sites and the organelle interaction network in plants

Functional regulation and structural maintenance of the different organelles in plants contribute directly to plant development, reproduction and stress responses. To ensure these activities take place effectively, cells have evolved an interconnected network amongst various subcellular compartments, regulating rapid signal transduction and the exchange of biomaterial. Many proteins that regulate membrane connections have recently been identified in plants, and this is the first step in elucidating both the mechanism and function of these connections. Amongst all organelles, the endoplasmic reticulum is the key structure, which likely links most of the different subcellular compartments through membrane contact sites (MCS) and the ER-PM contact sites (EPCS) have been the most intensely studied in plants. However, the molecular composition and function of plant MCS are being found to be different from other eukaryotic systems. In this article, we will summarise the most recent advances in this field and discuss the mechanism and biological relevance of these essential links in plants.

Investigating Plant Protein-Protein Interactions Using FRET-FLIM with a Focus on the Actin Cytoskeleton

The study of protein-protein interactions is fundamental to understanding how actin-dependent processes are controlled through the regulation of actin-binding proteins by their interactors. FRET-FLIM (Förster resonance energy transfer-fluorescence lifetime imaging microscopy) is a sensitive bioimaging method to detect protein-protein interactions in living cells through measurement of FRET, facilitated by the interactions of fluorophore-tagged fusion protein. As a sensitive and noninvasive method for the spatiotemporal visualization of dynamic protein-protein interactions, FRET-FLIM holds several advantages over other methods of protein interaction assays. FRET-FLIM has been widely employed to characterize many plant protein interactions, including interactions between actin-regulatory proteins and their binding partners. As we increasingly understand the plant actin cytoskeleton to coordinate a diverse number of complex functions, the study of actin-regulatory proteins and their interactors becomes increasingly technically challenging. Sophisticated and sensitive in vivo methods such as FRET-FLIM are likely to be crucial to the study of protein-protein interactions as more complex and challenging hypotheses are addressed.

Nuclear movement in growing Arabidopsis root hairs involves both actin filaments and microtubules

Nuclear migration during growth and development is a conserved phenomenon among many eukaryotic species. In Arabidopsis, movement of the nucleus is important for root hair growth, but the detailed mechanism behind this movement is not well known. Previous studies in different cell types have reported that the myosin XI-I motor protein is responsible for this nuclear movement by attaching to the nuclear transmembrane protein complex WIT1/WIT2. Here, we analyzed nuclear movement in growing root hairs of wild-type, myosin xi-i, and wit1 wit2 Arabidopsis lines in the presence of actin and microtubule-disrupting inhibitors to determine the individual effects of actin filaments and microtubules on nuclear movement. We discovered that forward nuclear movement during root hair growth can occur in the absence of myosin XI-I, suggesting the presence of an alternative actin-based mechanism that mediates rapid nuclear displacements. By quantifying nuclear movements with high temporal resolution during the initial phase of inhibitor treatment, we determined that microtubules work to dampen erratic nuclear movements during root hair growth. We also observed microtubule-dependent backwards nuclear movement when actin filaments were impaired in the absence of myosin XI-I, indicating the presence of complex interactions between the cytoskeletal arrays during nuclear movements in growing root hairs.

Encoding, transmission, decoding, and specificity of calcium signals in plants

Calcium acts as a signal and transmits information in all eukaryotes. Encoding machinery consisting of calcium channels, stores, buffers, and pumps can generate a variety of calcium transients in response to external stimuli, thus shaping the calcium signature. Mechanisms for the transmission of calcium signals have been described, and a large repertoire of calcium binding proteins exist that can decode calcium signatures into specific responses. Whilst straightforward as a concept, mysteries remain as to exactly how such information processing is biochemically implemented. Novel developments in imaging technology and genetically encoded sensors (such as calcium indicators), in particular for multi-signal detection, are delivering exciting new insights into intra- and intercellular calcium signaling. Here, we review recent advances in characterizing the encoding, transmission, and decoding mechanisms, with a focus on long-distance calcium signaling. We present technological advances and computational frameworks for studying the specificity of calcium signaling, highlight current gaps in our understanding and propose techniques and approaches for unravelling the underlying mechanisms.