牛痘相关激酶2在神经发育过程中小胶质细胞介导的突触消除中起重要作用

在出生后的神经发育过程中,大脑神经回路的建立需要小胶质细胞消除过度的突触连接。小胶质细胞突触调节缺失已被证实与自闭症、精神分裂症和智力残疾等神经发育障碍有关。小胶质细胞中表达的牛痘相关激酶2(VRK2)可能可以激活突触消除的过程。在VRK2缺陷小鼠(VRK2KO)中,可以观察到小胶质细胞内突触前位点的减少。而且,在出生后第2周,VRK2KO小鼠的突触前位点和突出的数量都出现了异常增加。这些差异并没有持续到成年期。尽管突触数量的增加最终结局与正常小鼠无异,但成年VRK2KO小鼠在社交行为、情境恐惧记忆和空间记忆中表现出行为缺陷。

GIGANTEA and EARLY FLOWERING 4 in Arabidopsis Exhibit Differential Phase-Specific Genetic Influences over a Diurnal Cycle

The endogenous circadian clock regulates many physiological processes related to plant survival and adapt- ability. GIGANTEA （GI）, a clock-associated protein, contributes to the maintenance of circadian period length and ampli- tude, and also regulates flowering time and hypocotyl growth in response to day length. Similarly, EARLY FLOWERING 4 （ELF4）, another clock regulator, also contributes to these processes. However, little is known about either the genetic or molecular interactions between GI and ELF4 in Arabidopsis. In this study, we investigated the genetic interactions between GI and ELF4 in the regulation of circadian clock-controlled outputs. Our mutant analysis shows that GI is epistatic to ELF4 in flowering time determination, while ELF4 is epistatic to GI in hypocotyl growth regulation. Moreover, GI and ELF4 have a synergistic or additive effect on endogenous clock regulation. Gene expression profiling of gi, elf4, and gi elf4 mutants further established that GI and ELF4 have differentially dominant influences on circadian physiological outputs at dusk and dawn, respectively. This phasing of GI and ELF4 influences provides a potential means to achieve diversity in the regulation of circadian physiological outputs, including flowering time and hypocotyl growth.

Molecular Mechanism of the Specificity of Protein Import into Chloroplasts and Mitochondria in Plant Cells

Plants possess both types of endosymbiotic organelles, chloroplasts and mitochondria. Transit peptides and presequences function as signal sequences for specific import into chloroplasts and mitochondria, respectively. However, how these highly similar signal sequences confer the protein import specificity remains elusive. Here, we show that mitochondrial- or chloroplast-specific import involves two distinct steps, specificity determination and translocation across envelopes, which are mediated by the N-terminal regions and functionally interchangeable C-terminal regions, respectively, of transit peptides and presequences. A domain harboring multiple-arginine and hydrophobic sequence motifs in the N-terminal regions of presequences was identified as the mitochondrial specificity factor. The presence of this domain and the absence of arginine residues in the N-terminal regions of otherwise common targeting signals confers specificity of protein import into mitochondria and chloroplasts, respectively. AtToc159, a chloroplast import receptor, also contributes to determining chloroplast import specificity. We propose that common ancestral sequences were functionalized into mitochondrial- and chloroplast-specific signal sequences by the presence and absence, respectively, of multiple-arginine and hydrophobic sequence motifs in the N-terminal region.

Dynamic spatial reorganization of BSK1 complexes in the plasma membrane underpins signal-specific activation for growth and immunity

Growth and immunity are opposing processes that compete for cellular resources,and proper resource allocation is crucial for plant survival.BSK1 plays a key role in the regulation of both growth and immunity by associating with BRI1 and FLS2,respectively.However,it remains unclear how two antagonistic signals co-opt BSK1 to induce signal-specific activation.Here we show that the dynamic spatial reorganiization of BSK1 within the plasma membrane underlies the mechanism of signal-specific activation for growth or immunity.Resting BSK1 localizes to membrane rafts as complexes.Unlike BSK1-associated FLS2 and BRI1,flg22 or exogenous brassinosteroid(BR)treatment did not decrease BSK1 levels at the plasma membrane(PM)but rather induced BSK1 multimerization and dissociation from FLS2/BSK1 or BRI1/BSK1,respectively.Moreover,flg22-activated BSK1 translocated from membrane rafts to non-membrane-raft regions,whereas BR-activated BSK1 remained in membrane rafts.When applied together with flg22,BR suppressed various flg22-induced BSK1 activities such as BSK1 dissociation from FLS2/BSK1,BSK1 interaction with MAPKKK5,and BSK translocation together with MAPKKK5.Taken together,this study provides a unique insight into how the precise control of BSK1 spatiotemporal organization regulates the signaling specificity to balance plant growth and immunity.

Plant ABC Transporters Enable Many Unique Aspects of a Terrestrial Plant＇s Lifestyle

Terrestrial plants have two to four times more ATP-binding cassette （ABC） transporter genes than other organisms, including their ancestral microalgae. Recent studies found that plants harboring mutations in these transporters exhibit dramatic phenotypes, many of which are related to developmental processes and functions necessary for life on dry land. These results suggest that ABC transporters multiplied during evolution and assumed novel functions that allowed plants to adapt to terrestrial environmental conditions. Examining the literature on plant ABC transporters from this viewpoint led us to propose that diverse ABC transporters enabled many unique and essential aspects of a terrestrial plant＇s lifestyle, by transporting various compounds across specific membranes of the plant.

Cytochrome b5 Reductase 1 Triggers Serial Reactions that Lead to Iron Uptake in Plants

Rhizosphere acidification is essential for iron （Fe） uptake into plant roots. Plasma membrane （PM） H＊-ATPases play key roles in rhizosphere acidification. However, it is not fully understood how PM H＋-ATPase activity is regulated to enhance root Fe uptake under Fe-deficient conditions. Here, we present evidence that cytochrome b5 reductase 1 （CBR1） increases the levels of unsaturated fatty acids, which stimulate PM H＋-ATPase activity and thus lead to rhizosphere acidification. CBRl-overexpressing （CBRI-OX） Arabidopsis thaliana plants had higher levels of unsaturated fatty acids （18：2 and 18：3）, higher PM H＊-ATPase activity, and lower rhizosphere pH than wild-type plants. By contrast, cbrl loss-of-function mutant plants showed lower levels of unsaturated fatty acids and lower PM H＊-ATPase activity but higher rhizosphere pH. Reduced PM H＊-ATPase activity in cbrl could be restored in vitro by addition of unsatu- rated fatty acids. Transcript levels of CBR1, fatty acids desaturase 2 （FAD2）, and fatty acids desaturase 3 （FAD3） were increased under Fe-deficient conditions. We propose that CBR1 has a crucial role in increasing the levels of unsaturated fatty acids, which activate the PM H＊-ATPase and thus reduce rhizosphere pH. This reaction cascade ultimately promotes root Fe uptake.

Interactions between Transmembrane Helices within Monomers of the Aquaporin AtPIP2;1 Play a Crucial Role in Tetramer Formation

Aquaporin （AQP） is a water channel protein found in various subcellular membranes of both prokaryotic and eukaryotic cells. The physiological functions of AQPs have been elucidated in many organisms. However, understanding their biogenesis remains elusive, particularly regarding how they assemble into tetramers. Here, we investigated the amino acid residues involved in the tetramer formation of the Arabidopsis plasma membrane AQP AtPIP2;1 using extensive amino acid substitution mutagenesis. The mutant proteins V41A/ E44A, F51A/L52A, F87A/191A, F92A/193A, V95A/Y96A, and H216A/L217A, harboring alanine substitutions in the transmembrane （TM） helices of AtPIP2;1 polymerized into multiple oligomeric complexes with a vari- able number of subunits greater than four. Moreover, these mutant proteins failed to traffic to the plasma membrane, instead of accumulating in the endoplasmic reticulum （ER）. Structure-based modeling revealed that these residues are largely involved in interactions between TM helices within monomers. These results suggest that inter-TM interactions occurring both within and between monomers play crucial roles in tetramer formation in the AtPIP2;1 complex. Moreover, the assembly of AtPIP2;1 tetramers is critical for their trafficking from the ER to the plasma membrane, as well as water permeability.

The A/ENTH Domain-Containing Protein AtECA4 Is an Adaptor Protein Involved in Cargo Recycling from the trans-Golgi Network/Early Endosome to the Plasma Membrane

Endocytosis and subsequent trafficking pathways are crucial for regulating the activity of plasma membrane-localized proteins. Depending on cellular and physiological conditions, the internalized cargoes are sorted at （and transported from） the trans-Golgi network/early endosome （TGN/EE） to the vacuole for degradation or recycled back to the plasma membrane. How this occurs at the molecular level remains largely elusive. Here, we provide evidence that the ENTH domain-containing protein AtECA4 plays a crucial role in recycling cargoes from the TGN/EE to the plasma membrane in Arabidopsis thaliana. AtECA4：sGFP primarily localized to the TGN/EE and plasma membrane （at low levels）. Upon NaCI or mannitol treatment, AtECA4：sGFP accumulated at the TGN/EE at an early time point but was released from the TGN/EE to the cytosol at later time points. The ateca4mutant showed higher resistance to osmotic stress and more sensitive to exogenous abscisic acid （ABA） than the wild type, as well as increased expres- sion of ABA-inducible genes RD29A and RD29B. Consistently, ABCG25, a plasma membrane-localized ABA exporter, accumulated at the prevacuolar compartment in ateca4, indicating a defect in recycling to the plasma membrane. However, the role of AtECA4 in cargo recycling is not specific to ABCG25, as it also functions in the recycling of BRII. These results suggest that AtECA4 plays a crucial role in the recycling of endocytosed cargoes from the TGN/EE to the plasma membrane.

Arabidopsis BAG1 Functions as a Cofactor in Hsc70-Mediated Proteasomal Degradation of Unimported Plastid Proteins

Dear Editor,Eliminating misfolded or mistargeted proteins is crucial for cell viability because these proteins accumulate as non-specific aggregates, which can be toxic to the cell （Lee et al., 2009; Sroka et al., 2009）. Previously, we have shown that in ppi2 （plastid protein import 2） mutant plants, the transcript levels of Hsc70-4 （one isoform of the Hsc70 family） and CHIP （an E3 ligase） were highly upregulated, which ultimately plays crucial roles in proteasomal degradation of unimported plastid proteins （Lee et al., 2009）. We also found that, along with those of Hsc70-4 and CHIP, the transcript level of AtBAG1 （Arabidopsis thaliana Bcl2-associated athanogene 1） in the ppi2 mutant was 2.38-fold higher than that in the wild-type （Lee et al., 2009）.

Liquid-Liquid Phase Transition as a New Means of Protein Targeting in Chloroplasts

The chloroplast is a unique organelle in that it possesses an intra-organellar membrane-enclosed structure called thylakoid,which is central to photosynthesis.Thus,the timely expression and correct localization of thylakoid proteins are of utmost importance to plant growth and development(Lee et al.,2017;New et al.,2018).Biogenesis of thylakoid proteins is highly complex;they are encoded by two different genomes,nuclear and chloroplast genomes,and translated in the cytosol and chloroplasts,respectively.Nuclear-encoded thylakoid proteins are first imported into the stroma of chloroplasts before they are targeted to thylakoids.In the stroma,multiple sorting mechanisms exist to ensure import or insertion of thylakoid luminal or membrane proteins,respectively(Lee et al.,2017).Approximately 50%of thylakoid luminal proteins are imported through the cpTAT(chloroplast twin-arginine translocation)pathway,which is also present in bacteria and plant mitochondria(Cline,2015;Schafer et al.f 2020).