Keratins and the plakin family cytolinker proteins control the length of epithelial microridge protrusions.

Actin filaments and microtubules create diverse cellular protrusions, but intermediate filaments, the strongest and most stable cytoskeletal elements, are not known to directly participate in the formation of protrusions. Here we show that keratin intermediate filaments directly regulate the morphogenesis of microridges, elongated protrusions arranged in elaborate maze-like patterns on the surface of mucosal epithelial cells. We found that microridges on zebrafish skin cells contained both actin and keratin filaments. Keratin filaments stabilized microridges, and overexpressing keratins lengthened them. Envoplakin and periplakin, plakin family cytolinkers that bind F-actin and keratins, localized to microridges, and were required for their morphogenesis. Strikingly, plakin protein levels directly dictate microridge length. An actin-binding domain of periplakin was required to initiate microridge morphogenesis, whereas periplakin-keratin binding was required to elongate microridges. These findings separate microridge morphogenesis into distinct steps, expand our understanding of intermediate filament functions, and identify microridges as protrusions that integrate actin and intermediate filaments.

Cells adopt a wide array of irregular and bumpy shapes, which are scaffolded by an internal structure called the cytoskeleton. This network of filaments can deform the cell membrane the way tent poles frame a canvas. Cells contain three types of cytoskeleton elements (actin filaments, intermediate filaments, and microtubules), each with unique chemical and mechanical properties. One of the main roles of the cytoskeleton is to create protrusions, a range of structures that 'stick out' of a cell to allow movement and interactions with the environment. Both actin filaments and microtubules help form protrusions, but the role of intermediate filaments remains unclear. Microridges are a type of protrusion found on cells covered by mucus, for instance on the surface of the eye, inside the mouth, or on fish skin. These small bumps are organised on the membrane of a cell in fingerprint-like arrangements. Scientists know that actin networks are necessary for microridges to form; yet, many structures supported by actin filaments are not stable over time, suggesting that another component of the cytoskeleton might be lending support. Intermediate filaments are the strongest, most stable type of cytoskeleton element, and they can connect to actin filaments via linker proteins. However, research has yet to show that this kind of cooperation happens in any membrane protrusion. Here, Inaba et al. used high-resolution microscopy to monitor microridge development in the skin of live fish. In particular, they focused on a type of intermediate filaments known as keratin filaments. This revealed that, inside microridges, the keratin and actin networks form alongside each other, with linker proteins called Envoplakin and Periplakin connecting the two structures together. Genetic experiments revealed that Envoplakin and Periplakin must attach to actin for microridges to start forming. However, the two proteins bind to keratin for protrusions to grow. This work therefore highlights how intermediate filaments and linker proteins contribute to the formation of these structures. Many tissues must be covered in mucus to remain moist and healthy. As microridges likely contribute to mucus retention, the findings by Inaba et al. may help to better understand how disorders linked to problems in mucus emerge.

Installation of authentic BicA and SbtA proteins to the chloroplast envelope membrane is achieved by the proteolytic cleavage of chimeric proteins in Arabidopsis.

To improve the photosynthetic performance of C3 plants, installing cyanobacterial bicarbonate transporters to the chloroplast inner envelope membrane (IEM) has been proposed for years. In our previous study, we successfully introduced chimeric cyanobacterial sodium-dependent bicarbonate transporters, BicA or SbtA, to the chloroplast IEM of Arabidopsis. However, the installation of authentic BicA and SbtA to the chloroplast IEM has not been achieved yet. In this study, we examined whether or not tobacco etch virus (TEV) protease targeted within chloroplasts can cleave chimeric proteins and produce authentic bicarbonate transporters. To this end, we constructed a TEV protease that carried the transit peptide and expressed it with chimeric BicA or SbtA proteins containing a TEV cleavage site in planta. Chimeric proteins were cleaved only when the TEV protease was co-expressed. The authentic forms of hemagglutinin-tagged BicA and SbtA were detected in the chloroplast IEM. In addition, cleavage of chimeric proteins at the TEV recognition site seemed to occur after the targeting of chimeric proteins to the chloroplast IEM. We conclude that the cleavage of chimeric proteins within chloroplasts is an efficient way to install authentic bicarbonate transporters to the chloroplast IEM. Furthermore, a similar approach can be applied to other bacterial plasma membrane proteins.

Transport of the outer dynein arm complex to cilia requires a cytoplasmic protein Lrrc6.

Lrrc6 encodes a cytoplasmic protein that is expressed specifically in cells with motile cilia including the node, trachea and testes of the mice. A mutation of Lrrc6 has been identified in human patients with primary ciliary dyskinesia (PCD). Mutant mice lacking Lrrc6 show typical PCD defects such as hydrocephalus and laterality defects. We found that in the absence of Lrrc6, the morphology of motile cilia remained normal, but their motility was completely lost. The 9 + 2 arrangement of microtubules remained normal in Lrrc6(-/-) mice, but the outer dynein arms (ODAs), the structures essential for the ciliary beating, were absent from the cilia. In the absence of Lrrc6, ODA proteins such as DNAH5, DNAH9 and IC2, which are assembled in the cytoplasm and transported to the ciliary axoneme, remained in the cytoplasm and were not transported to the ciliary axoneme. The IC2-IC1 interaction, which is the first step of ODA assembly, was normal in Lrrc6(-/-) mice testes. Our results suggest that ODA proteins may be transported from the cytoplasm to the cilia by an Lrrc6-dependent mechanism.

Isolation and gene expression analysis of a papain-type cysteine protease in thermogenic skunk cabbage (Symplocarpus renifolius).

Skunk cabbage (Symplocarpus renifolius) spadices contain abundant transcripts for cysteine protease (CP). From thermogenic spadices, we isolated SrCPA, a highly expressed CP gene that encoded a papain-type CP. SrCPA is structurally similar to other plant CPs, including the senescence-associated CPs found in aroids. The expression of SrCPA increased during floral development, and was observed in all floral tissues except for the stamens.

The gene expression landscape of thermogenic skunk cabbage suggests critical roles for mitochondrial and vacuolar metabolic pathways in the regulation of thermogenesis.

Floral thermogenesis has been described in several plant species. Because of the lack of comprehensive gene expression profiles in thermogenic plants, the molecular mechanisms by which floral thermogenesis is regulated remain to be established. We examined the gene expression landscape of skunk cabbage (Symplocarpus renifolius) during thermogenic and post-thermogenic stages and identified expressed sequence tags from different developmental stages of the inflorescences using super serial analysis of gene expression (SuperSAGE). In-depth analysis suggested that cellular respiration and mitochondrial functions are significantly enhanced during the thermogenic stage. In contrast, genes involved in stress responses and protein degradation were significantly up-regulated during post-thermogenic stages. Quantitative comparisons indicated that the expression levels of genes involved in cellular respiration were higher in thermogenic spadices than in Arabidopsis inflorescences. Thermogenesis-associated genes seemed to be expressed abundantly in the peripheral tissues of the spadix. Our results suggest that cellular respiration and mitochondrial metabolism play key roles in heat production during floral thermogenesis. On the other hand, vacuolar cysteine protease and other degradative enzymes seem to accelerate senescence and terminate thermogenesis in the post-thermogenic stage.