Recent advances in plant membrane-bound transcription factor research: Emphasis on intracellular movement

Transcription factors constitute numerous signal transduction networks and play a central role in gene expression regulation. Recent studies have shown that a limited portion of transcription factors are anchored in the cellular membrane, storing as dormant forms. Upon exposure to environmental and developmental cues, these transcription factors are released from the membrane and translocated to the nucleus, where they regulate associated target genes. As this process skips both transcriptional and translational regulations, it guarantees prompt response to external and internal signals. Membrane- bound transcription factors （MTFs） undergo several unique steps that are not involved in the action of canonical nuclear transcription factors： proteolytic processing and intracellular movement. Recently, alternative splicing has also emerged as a mechanism to liberate MTFs from the cellular membranes, establishing an additional activation scheme independent of proteolytic processing. Multiple layers of MTF regulation add complexity to transcriptional regulatory scheme and ensure elaborate action of MTFs. In this review, we provide an overview of recent findings on MTFs in plants and highlight the molecular mechanisms underlying MTF liberation from cellular membranes with an emphasis on intracellular movement.

A novel semi-dominant allele of the transmembrane NAC transcription factor ZmNTL2 reduces the size of multiple maize organs

Maize plant architecture influences planting density and,in turn,grain yield.Most of the plant architecture-related traits can be described as organ size.We describe a miniature maize mutant,Tiny plant 4(Tip4),which exhibits reduced size of multiple organs and exhibits a semi-dominant monofactorial inheritance characteristic.Positional cloning confirmed that a 4-bp deletion in the NAC TF with transmembrane motif 1-Like(NTL)gene ZmNTL2,denoted as ZmNTL2^(Δ),confers the Tip4 mutation.qRT-PCR showed that ZmNTL2 was expressed in all tested tissues.ZmNTL2 functions as a transcriptional activator and is located in both the nucleus and biomembranes.The mutation does not affect the mRNA abundance of ZmNTL2 locus,but it does result in the loss of transmembrane domain and confines the ZmNTL2^(Δ)protein to the nucleus.Knocking out ZmNTL2 has no effect on maize organ size development,indicating that the 4-bp deletion might be a gain-of-function mutation in organ size regulation.Combining transcriptome sequencing with cytokinin and auxin content determination suggests that the decreased organ size may be possibly mediated by changes in hormone homeostasis.

Relationship between interleukin-1 type I and 2 receptor gene polymorphisms and the expression level of membrane-bound receptors

The biological activity of the multifunctional cytokine interleukin-1 （IL-1） is mediated by its receptors. The aim of this study was to determine if an association exists between single nucleotide polymorphisms （SNPs） in the IL-1 type I and 2 receptor genes （ILIR1 and ILIR2） and the expression level of membrane-bound ILIRs on subpopulations of mononuclear cells or serum levels of soluble IL-1 receptors. It was observed that healthy individuals with the genotype TT in SNP rs2234650.C〉T had a lower percentage of intact CD14＋ monocytes expressing ILIR1 on their surface. The SNP rs4141134-T〉C in ILIR2 has also been associated with the percentage of intact CD3＋ T cells expressing ILIR2. Furthermore, individuals carrying the CC allele of SNP rs4141134.T〉C and the TT allele of SNP rs2071008-T〉G in ILIR2 had a lower density of ILIR2s on the surface of CD14＋ monocytes in lipopolysaccharide （LPS）-stimulated PBMC cultures. In summary, this study demonstrated that IL-1 receptor gene polymorphisms could be one of the factors influencing the expression of membrane-bound IL-1 receptors （ILIR） on immunocompetent cells.

Antibody Therapies Targeting Complex Membrane Proteins

In analyses of protein families that may serve as drug targets,membrane-associated G-protein-coupled receptors(GPCRs)dominate,followed by ion channels,transporters,and—to a lesser extent—membrane-bound enzymes.However,various challenges put such membrane proteins among key groups of underutilized opportunities for the application of therapeutic antibodies.Antibodies hold the promise of exquisite specificity,as they are able to target even specific conformations of a particular membrane protein,as well as adaptability through engineering into various antibody formats.However,the ease of raising and isolating specific,effective antibodies targeting membrane proteins depends on many factors.In particular,the generation of specific antibodies is easier when targeting larger,simpler,extracellular domains with greater uniqueness of amino acid sequence.The rareness of such ideal conditions is illustrated by the limited number of approved biologics for targeting GPCRs and other complex membrane proteins.Challenges in developing antibodies to complex membrane proteins such as GPCRs,ion channels,transporters,and membrane-bound enzymes can be addressed by the design of the antigen,antibody-generation strategies,lead optimization technologies,and antibody modalities.A better understanding of the membrane proteins being targeted would facilitate mechanism-based drug discovery.This review describes the advantages and challenges of targeting complex membrane proteins with antibodies and discusses the preparation of membrane protein antigens and antibody generation,illustrated by select examples of success.

mIgM-mediated splenic marginal zone B cells targeting of folic acid for immunological evasion

Folic acid is a fully oxidized synthetic folate with high bioavailability and stability which has been extensively prescribed to prevent congenital disabilities.Here we revealed the immunosuppressive effect of folic acid by targeting splenic marginal zone B(MZB)cells.Folic acid demonstrates avid binding with the Fc domain of immunoglobulin M(IgM),targeting IgM positive MZB cells in vivo to destabilize IgM-B cell receptor(BCR)complex and block immune responses.The induced anergy of MZB cells by folic acid provides an immunological escaping window for antigens.Covalent conjugation of folic acid with therapeutic proteins and antibodies induces immunological evasion to mitigate the production of anti-drug antibodies,which is a major obstacle to the long-term treatment of biologics by reducing curative effects and/or causing adverse reactions.Folic acid acts as a safe and effective immunosuppressant via IgM-mediated MZB cells targeting to boost the clinical outcomes of biologics by inhibiting the production of anti-drug antibodies,and also holds the potential to treat other indications that adverse immune responses need to be transiently shut off.

Functional expression and regulation of eukaryotic cytochrome P450 enzymes in surrogate microbial cell factories

Cytochrome P450(CYP)enzymes play crucial roles during the evolution and diversification of ancestral monocel-lular eukaryotes into multicellular eukaryotic organisms due to their essential functionalities including catalysis of housekeeping biochemical reactions,synthesis of diverse metabolites,detoxification of xenobiotics,and con-tribution to environmental adaptation.Eukaryotic CYPs with versatile functionalities are undeniably regarded as promising biocatalysts with great potential for biotechnological,pharmaceutical and chemical industry applica-tions.Nevertheless,the modes of action and the challenges associated with these membrane-bound proteins have hampered the effective utilization of eukaryotic CYPs in a broader range.This review is focused on comprehen-sive and consolidated approaches to address the core challenges in heterologous expression of membrane-bound eukaryotic CYPs in different surrogate microbial cell factories,aiming to provide key insights for better studies and applications of diverse eukaryotic CYPs in the future.We also highlight the functional significance of the previously underrated cytochrome P450 reductases(CPRs)and provide a rational justification on the progression of CPR from auxiliary redox partner to function modulator in CYP catalysis.

MBOAT1 homozygous missense variant causes nonobstructive azoospermia

Nonobstructive azoospermia(NOA)is a common cause of infertility and is defined as the complete absence of sperm in ejaculation due to defective spermatogenesis.The aim of this study was to identify the genetic etiology of NOA in an infertile male from a Chinese consanguineous family.A homozygous missense variant of the membrane-bound O-acyltransferase domain-containing 1(MBOAT1)gene(c.770C>T,p.Thr257Met)was found by whole-exome sequencing(WES).Bioinformatic analysis also showed that this variant was a pathogenic variant and that the amino acid residue in MBOAT1 was highly conserved in mammals.Quantitative polymerase chain reaction(Q-PCR)analysis showed that the mRNA level of MBOAT1 in the patient was 22.0%lower than that in his father.Furthermore,we screened variants of MBOAT1 in a broader population and found an additional homozygous variant of the MBOAT1 gene in 123 infertile men.Our data identified homozygous variants of the MBOAT1 gene associated with male infertility.This study will provide new insights for researchers to understand the molecular mechanisms of male infertility and will help clinicians make accurate diagnoses.

α1-COP modulates plasmodesmata function through sphingolipid enzyme regulation

Callose,aβ-1,3-glucan plant cell wall polymer,regulates symplasmic channel size at plasmodesmata(PD)and plays a crucial role in a variety of plant processes.However,elucidating the molecular mechanism of PD callose homeostasis is limited.We screened and identified an Arabidopsis mutant plant with excessive callose deposition at PD and found that the mutated gene wasα1-COP,a member of the coat protein I(COPI)coatomer complex.We report that loss of function ofα1-COP elevates the callose accumulation at PD by affecting subcellular protein localization of callose degradation enzyme Pd BG2.This process is linked to the functions of ERH1,an inositol phosphoryl ceramide synthase,and glucosylceramide synthase through physical interactions with theα1-COP protein.Additionally,the loss of function ofα1-COP alters the subcellular localization of ERH1 and GCS proteins,resulting in a reduction of Glc Cers and Glc HCers molecules,which are key sphingolipid(SL)species for lipid raft formation.Our findings suggest thatα1-COP protein,together with SL modifiers controlling lipid raft compositions,regulates the subcellular localization of GPI-anchored PDBG2 proteins,and hence the callose turnover at PD and symplasmic movement of biomolecules.Our findings provide the first key clue to link the COPI-mediated intracellular trafficking pathway to the callose-mediated intercellular signaling pathway through PD.