Hydrophobic profiles of the tail anchors in SLMAP dictate subcellular targeting

The Cryptococcus neoformans STRIPAK complex controls genome stability, sexual development, and virulence

The eukaryotic serine/threonine protein phosphatase PP2A is a heterotrimeric enzyme composed of a scaffold A subunit, a regulatory B subunit, and a catalytic C subunit. Of the four known B subunits, the B''' subunit (known as striatin) interacts with the multi-protein striatin-interacting phosphatase and kinase (STRIPAK) complex. Orthologs of STRIPAK components were identified in C. neoformans , namely PP2AA/Tpd3, PP2AC/Pph22, PP2AB'''/Far8, STRIP/Far11, SLMAP/Far9, and Mob3. Structural modeling, protein domain analysis, and detected protein-protein interactions suggest C. neoformans STRIPAK is assembled similarly to the human and fungal orthologs. Here, STRIPAK components Pph22, Far8, and Mob3 were functionally characterized. Whole-genome sequencing revealed that mutations in STRIPAK complex subunits lead to increased segmental and chromosomal aneuploidy, suggesting STRIPAK functions in maintaining genome stability. We demonstrate that PPH22 is a haploinsufficient gene: heterozygous PPH22/pph22 Δ mutant diploid strains exhibit defects in hyphal growth and sporulation and have a significant fitness disadvantage when grown in competition against a wild-type diploid. Deletion mutants pph22 Δ, far8 Δ, and mob3 Δ exhibit defects in mating and sexual differentiation, including impaired hyphae, basidia, and basidiospore production. Loss of either PPH22 or FAR8 leads to growth defects at 30⁰C, severely reduced growth at elevated temperature, abnormal cell morphology, and impaired virulence. The pph22 Δ and far8 Δ mutants are also unable to grow in the presence of the calcineurin inhibitors cyclosporine A or FK506, and thus these mutations are synthetically lethal with loss of calcineurin activity. Conversely, mob3 Δ mutants display increased thermotolerance, capsule production, and melanization, and are hypervirulent in a murine infection model. Taken together, these findings reveal that the C. neoformans STRIPAK complex plays an important role in genome stability, vegetative growth, sexual development, and virulence in this prominent human fungal pathogen.

Deletion of Sarcolemmal Membrane-Associated Protein Isoform 3 (SLMAP3) in Cardiac Progenitors Delays Embryonic Growth of Myocardium without Affecting Hippo Pathway

The slmap gene is alternatively spliced to generate many isoforms that are abundant in developing myocardium. The largest protein isoform SLMAP3 is ubiquitously expressed and has been linked to cardiomyopathy, Brugada syndrome and Hippo signaling. To examine any role in cardiogenesis, mice homozygous for floxed slmap allele were crossed with Nkx2.5-cre mice to nullify its expression in cardiac progenitors. Targeted deletion of the slmap gene resulted in the specific knockout (KO) of the SLMAP3 (~91 KDa) isoform without any changes in the expression of the SLMAP2 (~43 kDa) or the SLMAP1 (~35 kDa) isoforms which continued to accumulate to similar levels as seen in Wt embryonic hearts. The loss of SLMAP3 from cardiac progenitors resulted in decreased size of the developing embryonic hearts evident at E9.5 to E16.5 with four small chambers and significantly thinner left ventricles. The proliferative capacity assessed with the phosphorylation of histone 3 or with Ki67 in E12.5 hearts was not significantly altered due to SLMAP3 deficiency. The size of embryonic cardiomyocytes, marked with anti-Troponin C, revealed significantly smaller cells, but their hypertrophic response (AKT1 and MTOR1) was not significantly affected by the specific loss of SLMAP3 protein. Further, no changes in phosphorylation of MST1/2 or YAP were detected in SLMAP3-KO embryonic myocardium, ruling out any impact on Hippo signaling. Rat embryonic cardiomyocytes express the three SLMAP isoforms and their knockdown (KD) with sh-RNA, resulted in decreased proliferation and enhanced senescence but without any impact on Hippo signaling. Collectively, these data show that SLMAP is critical for normal cardiac development with potential for the various isoforms to serve compensatory roles. Our data imply novel mechanisms for SLMAP action in cardiac growth independent of Hippo signaling.

Comprehensive analysis of RNA-binding protein SRSF2-dependent alternative splicing signature in malignant proliferation of colorectal carcinoma

Aberrant expression of serine/arginine-rich splicing factor 2 (SRSF2) can lead to tumorigenesis, but its molecular mechanism in colorectal cancer is currently unknown. Herein, we found SRSF2 to be highly expressed in human colorectal cancer (CRC) samples compared with normal tissues. Both in vitro and in vivo, SRSF2 significantly accelerated the proliferation of colon cancer cells. Using RNA-seq, we screened and identified 33 alternative splicing events regulated by SRSF2. Knockdown of SLMAP-L or CETN3-S splice isoform could suppress the growth of colon cancer cells, predicting their role in malignant proliferation of colon cancer cells. Mechanistically, the in vivo crosslinking immunoprecipitation assay demonstrated the direct binding of the RNA recognition motif of SRSF2 protein to SLMAP and CETN3 pre-mRNAs. SRSF2 activated the inclusion of SLMAP alternative exon 24 by binding to constitutive exon 25, while SRSF2 facilitated the exclusion of CETN3 alternative exon 5 by binding to neighboring exon 6. Knockdown of SRSF2, its splicing targets SLMAP-L, or CETN3-S caused colon cancer cells to arrest in G1 phase of the cell cycle. Rescue of SLMAP-L or CETN3-S splice isoform in SRSF2 knockdown colon cancer cells could effectively reverse the inhibition of cell proliferation by SRSF2 knockdown through mediating cell cycle progression. Importantly, the percentage of SLMAP exon 24 inclusion increased and CETN3 exon 5 inclusion decreased in CRC samples compared to paired normal samples. Collectively, our findings identify that SRSF2 dysregulates colorectal carcinoma proliferation at the molecular level of splicing regulation and reveal potential splicing targets in CRC patients.

Multiphase coalescence mediates Hippo pathway activation

The function of biomolecular condensates is often restricted by condensate dissolution. Whether condensates can be suppressed without condensate dissolution is unclear. Here, we show that upstream regulators of the Hippo signaling pathway form functionally antagonizing condensates, and their coalescence into a common phase provides a mode of counteracting the function of biomolecular condensates without condensate dissolution. Specifically, the negative regulator SLMAP forms Hippo-inactivating condensates to facilitate pathway inhibition by the STRIPAK complex. In response to cell-cell contact or osmotic stress, the positive regulators AMOT and KIBRA form Hippo-activating condensates to facilitate pathway activation. The functionally antagonizing SLMAP and AMOT/KIBRA condensates further coalesce into a common phase to inhibit STRIPAK function. These findings provide a paradigm for restricting the activity of biomolecular condensates without condensate dissolution, shed light on the molecular principles of multiphase organization, and offer a conceptual framework for understanding upstream regulation of the Hippo signaling pathway.

The Green Valley of Drosophila melanogaster Constitutive Heterochromatin: Protein-Coding Genes Involved in Cell Division Control

Constitutive heterochromatin represents a significant fraction of eukaryotic genomes (10% in Arabidopsis, 20% in humans, 30% in D. melanogaster, and up to 85% in certain nematodes) and shares similar genetic and molecular properties in animal and plant species. Studies conducted over the last few years on D. melanogaster and other organisms led to the discovery of several functions associated with constitutive heterochromatin. This made it possible to revise the concept that this ubiquitous genomic territory is incompatible with gene expression. The aim of this review is to focus the attention on a group of protein-coding genes resident in D. melanogaster constitutive of heterochromatin, which are implicated in different steps of cell division.

Specific Deletion of the FHA Domain Containing SLMAP3 Isoform in Postnatal Myocardium Has No Impact on Structure or Function

Sarcolemmal membrane-associated proteins (SLMAPs) belong to the superfamily of tail-anchored membrane proteins known to regulate diverse biological processes, including protein trafficking and signal transduction. Mutations in SLMAP have been linked to Brugada and defective sodium channel Nav1.5 shuttling. The SLMAP gene is alternatively spliced to generate numerous isoforms, broadly defined as SLMAP1 (~35 kDa), SLMAP2 (~45 kDa) and SLMAP3 (~80–95 kDa), which are highly expressed in the myocardium. The SLMAP3 isoform exhibits ubiquitous expression carrying an FHA domain and is believed to negatively regulate Hippo signaling to dictate cell growth/death and differentiation. Using the αMHC-MerCreMer-flox system to target the SLMAP gene, we specifically deleted the SLMAP3 isoform in postnatal mouse hearts without any changes in the expression of SLMAP1/SLMAP2 isoforms. The in vivo analysis of mice with SLMAP3 cardiac deficiency revealed no significant changes to heart structure or function in young or aged mice without or with isoproterenol-induced stress. SLMAP3-deficient hearts revealed no obvious differences in cardiac size, function or hypertrophic response. Further, the molecular analysis indicated that SLMAP3 loss had a minor impact on sodium channel (Nav1.5) expression without affecting cardiac electrophysiology in postnatal myocardium. Surprisingly, the loss of SLMAP3 did not impact Hippo signaling in postnatal myocardium. We conclude that the FHA domain-containing SLMAP3 isoform has no impact on Hippo signaling or sodium channels in postnatal myocardium, which is able to function and respond normally to stress in its absence. Whether SLMAP1/SMAP2 isoforms can compensate for the loss of SLMAP3 in the affairs of the postnatal heart remains to be determined.

STRIPAK, a Key Regulator of Fungal Development, Operates as a Multifunctional Signaling Hub

The striatin-interacting phosphatases and kinases (STRIPAK) multi subunit complex is a highly conserved signaling complex that controls diverse developmental processes in higher and lower eukaryotes. In this perspective article, we summarize how STRIPAK controls diverse developmental processes in euascomycetes, such as fruiting body formation, cell fusion, sexual and vegetative development, pathogenicity, symbiosis, as well as secondary metabolism. Recent structural investigations revealed information about the assembly and stoichiometry of the complex enabling it to act as a signaling hub. Multiple organellar targeting of STRIPAK subunits suggests how this complex connects several signaling transduction pathways involved in diverse cellular developmental processes. Furthermore, recent phosphoproteomic analysis shows that STRIPAK controls the dephosphorylation of subunits from several signaling complexes. We also refer to recent findings in yeast, where the STRIPAK homologue connects conserved signaling pathways, and based on this we suggest how so far non-characterized proteins may functions as receptors connecting mitophagy with the STRIPAK signaling complex. Such lines of investigation should contribute to the overall mechanistic understanding of how STRIPAK controls development in euascomycetes and beyond.

Association and dissociation between the mitochondrial Far complex and Atg32 regulate mitophagy

Mitophagy plays an important role in mitochondrial homeostasis. In yeast, the phosphorylation of the mitophagy receptor Atg32 by casein kinase 2 is essential for mitophagy. This phosphorylation is counteracted by the yeast equivalent of the STRIPAK complex consisting of the PP2A-like protein phosphatase Ppg1 and Far3-7-8-9-10-11 (Far complex), but the underlying mechanism remains elusive. Here we show that two subpopulations of the Far complex reside in the mitochondria and endoplasmic reticulum, respectively, and play distinct roles; the former inhibits mitophagy via Atg32 dephosphorylation, and the latter regulates TORC2 signaling. Ppg1 and Far11 form a subcomplex, and Ppg1 activity is required for the assembling integrity of Ppg1-Far11-Far8. The Far complex preferentially interacts with phosphorylated Atg32, and this interaction is weakened by mitophagy induction. Furthermore, the artificial tethering of Far8 to Atg32 prevents mitophagy. Taken together, the Ppg1-mediated Far complex formation and its dissociation from Atg32 are crucial for mitophagy regulation.

Quality control pathways of tail-anchored proteins

Tail-anchored (TA) proteins have an N-terminal domain in the cytosol and a C-terminal transmembrane domain anchored to a variety of organelle membranes. TA proteins are recognized by targeting factors at the transmembrane domain and C-terminal sequence and are guided to distinct membranes. The promiscuity of targeting sequences and the dysfunction of targeting pathways cause mistargeting of TA proteins. TA proteins are under surveillance by quality control pathways. For resident TA proteins at mitochondrial and ER membranes, intrinsic instability or stimuli induced degrons of the cytosolic and transmembrane domains are sensed by quality control factors to initiate degradation of TA proteins. These pathways are summarized as TA protein degradation-Cytosol (TAD-C) and TAD-Membrane (TAD-M) pathways. For mistargeted and a subset of solitary TA proteins at mitochondrial and peroxisomal membranes, a unique pathway has been revealed in recent years. Msp1/ATAD1 is an AAA-ATPase dually-localized to mitochondrial and peroxisomal membranes. It directly recognizes mistargeted and solitary TA proteins and dislocates them out of membrane. Dislocated substrates are subsequently ubiquitinated by the ER-resident Doa10 ubiquitin E3 ligase complex for degradation. We summarize and discuss the substrate recognition, dislocation and degradation mechanisms of the Msp1 pathway.

The SLMAP/Striatin complex: An emerging regulator of normal and abnormal cardiac excitation-contraction coupling

The excitation-contraction (E-C) module involves a harmonized correspondence between the sarcolemma and the sarcoplasmic reticulum. This is provided by membrane proteins, which primarily shape the caveolae, the T-tubule/Sarcoplasmic reticulum (TT/SR) junction, and the intercalated discs (ICDs). Distortion of either one of these structures impairs myocardial contraction, and subsequently translates into cardiac failure. Thus, detailed studies on the molecular cues of the E-C module are becoming increasingly necessary to pharmacologically eradicate cardiac failure Herein we reviewed the organization of caveolae, TT/SR junctions, and the ICDs in the heart, with special attention to the Sarcolemma Membrane Associated Protein (SLMAP) and striatin (STRN) in cardiac membranes biology and cardiomyocyte contraction. We emphasized on their in vivo and in vitro signaling in cardiac function/dysfunction. SLMAP is a cardiac membrane protein that plays an important role in E-C coupling and the adrenergic response of the heart. Similarly, STRN is a dynamic protein that is also involved in cardiac E-C coupling and ICD-related cardiomyopathies. Both SLMAP and STRN are linked to cardiac conditions, including heart failure, and their role in cardiomyocyte function was elucidated in our laboratory. They interact together in a protein complex that holds therapeutic potentials for cardiac dysfunction. This review is the first of its kind to conceptualize the role of the SLMAP/STRN complex in cardiac function and failure. It provides in depth information on the signaling of these two proteins and projects their interaction as a novel therapeutic target for cardiac failure.

psichomics: graphical application for alternative splicing quantification and analysis

Alternative pre-mRNA splicing generates functionally distinct transcripts from the same gene and is involved in the control of multiple cellular processes, with its dysregulation being associated with a variety of pathologies. The advent of next-generation sequencing has enabled global studies of alternative splicing in different physiological and disease contexts. However, current bioinformatics tools for alternative splicing analysis from RNA-seq data are not user-friendly, disregard available exon-exon junction quantification or have limited downstream analysis features. To overcome such limitations, we have developed psichomics, an R package with an intuitive graphical interface for alternative splicing quantification and downstream dimensionality reduction, differential splicing and gene expression and survival analyses based on The Cancer Genome Atlas, the Genotype-Tissue Expression project, the Sequence Read Archive project and user-provided data. These integrative analyses can also incorporate clinical and molecular sample-associated features. We successfully used psichomics in a laptop to reveal alternative splicing signatures specific to stage I breast cancer and associated novel putative prognostic factors.

STRIPAK, a highly conserved signaling complex, controls multiple eukaryotic cellular and developmental processes and is linked with human diseases

The striatin-interacting phosphatases and kinases (STRIPAK) complex is evolutionary highly conserved and has been structurally and functionally described in diverse lower and higher eukaryotes. In recent years, this complex has been biochemically characterized better and further analyses in different model systems have shown that it is also involved in numerous cellular and developmental processes in eukaryotic organisms. Further recent results have shown that the STRIPAK complex functions as a macromolecular assembly communicating through physical interaction with other conserved signaling protein complexes to constitute larger dynamic protein networks. Here, we will provide a comprehensive and up-to-date overview of the architecture, function and regulation of the STRIPAK complex and discuss key issues and future perspectives, linked with human diseases, which may form the basis of further research endeavors in this area. In particular, the investigation of bi-directional interactions between STRIPAK and other signaling pathways should elucidate upstream regulators and downstream targets as fundamental parts of a complex cellular network.

SLMAP3 isoform modulates cardiac gene expression and function

The sarcolemmal membrane associated proteins (SLMAPs) belong to the super family of tail anchored membrane proteins which serve diverse roles in biology including cell growth, protein trafficking and ion channel regulation. Mutations in human SLMAP have been linked to Brugada syndrome with putative deficits in trafficking of the sodium channel (Na_v1.5) to the cell membrane resulting in aberrant electrical activity and heart function. Three main SLMAP isoforms (SLMAP1 (35 kDa), SLMAP2 (45 kDa), and SLMAP3 (91 kDa)) are expressed in myocardium but their precise role remains to be defined. Here we generated transgenic (Tg) mice with cardiac-specific expression of the SLMAP3 isoform during postnatal development which present with a significant decrease (20%) in fractional shortening and (11%) in cardiac output at 5 weeks of age. There was a lack of any notable cardiac remodeling (hypertrophy, fibrosis or fetal gene activation) in Tg hearts but the electrocardiogram indicated a significant increase (14%) in the PR interval and a decrease (43%) in the R amplitude. Western blot analysis indicated a selective and significant decrease (55%) in protein levels of Na_v1.5 while 45% drop in its transcript levels were detectable by qRT-PCR. Significant decreases in the protein and transcript levels of the calcium transport system of the sarcoplasmic reticulum (SERCA2a/PLN) were also evident in Tg hearts. These data reveal a novel role for SLMAP3 in the selective regulation of important ion transport proteins at the level of gene expression and suggest that it may be a unique target in cardiovascular function and disease.

SLMAP-3 is downregulated in human dilated ventricles and its overexpression promotes cardiomyocyte response to adrenergic stimuli by increasing intracellular calcium

Structural dilation of cardiomyocytes (CMs) imposes a decline in cardiac performance that precipitates cardiac failure and sudden death. Since membrane proteins are implicated in dilated cardiomyopathy and heart failure, we evaluated the expression of the sarcolemmal membrane-associated protein (SLMAP) in dilated cardiomyopathy and its effect on CM contraction. We found that all 3 SLMAP isoforms (SLMAP-1, -2, and -3) are expressed in CMs and are downregulated in human dilated ventricles. Knockdown of SLMAPs in cultured CMs transduced with recombinant adeno-associated viral particles releasing SLMAP-shRNA precipitated reduced spontaneous contractile rate that was not fully recovered in SLMAP-depleted CMs challenged with isoproterenol (ISO), thus phenotypically mimicking heart failure performance. Interestingly, the overexpression of the SLMAP-3 full-length isoform induced a positive chronotropic effect in CMs that was more pronounced in response to ISO insult (vs. ISO-treated naïve CMs). Confocal live imaging showed that H9c2 cardiac myoblasts overexpressing SLMAP-3 exhibit a higher intracellular calcium transient peak when treated with ISO (vs. ISO-treated cells carrying a control adeno-associated viral particle). Proteomics revealed that SLMAP-3 interacts with the regulator of CM contraction, striatin. Collectively, our data demonstrate that SLMAP-3 is a novel regulator of CM contraction rate and their response to adrenergic stimuli. Loss of SLMAPs phenotypically mimics cardiac failure and crystallizes SLMAPs as predictive of dilated cardiomyopathy and heart failure.

Homeostatic Control of Hpo/MST Kinase Activity through Autophosphorylation-Dependent Recruitment of the STRIPAK PP2A Phosphatase Complex

The Hippo pathway controls organ size and tissue homeostasis through a kinase cascade leading from the Ste20-like kinase Hpo (MST1/2 in mammals) to the transcriptional coactivator Yki (YAP/TAZ in mammals). Whereas previous studies have uncovered positive and negative regulators of Hpo/MST, how they are integrated to maintain signaling homeostasis remains poorly understood. Here, we identify a self-restricting mechanism whereby autophosphorylation of an unstructured linker in Hpo/MST creates docking sites for the STRIPAK PP2A phosphatase complex to inactivate Hpo/MST. Mutation of the phospho-dependent docking sites in Hpo/MST or deletion of Slmap, the STRIPAK subunit recognizing these docking sites, results in constitutive activation of Hpo/MST in both Drosophila and mammalian cells. In contrast, autophosphorylation of the Hpo/MST linker at distinct sites is known to recruit Mats/MOB1 to facilitate Hippo signaling. Thus, multisite autophosphorylation of Hpo/MST linker provides an evolutionarily conserved built-in molecular platform to maintain signaling homeostasis by coupling antagonistic signaling activities.

Mechanisms of Tail-Anchored Membrane Protein Targeting and Insertion

Proper localization of membrane proteins is essential for the function of biological membranes and for the establishment of organelle identity within a cell. Molecular machineries that mediate membrane protein biogenesis need to not only achieve a high degree of efficiency and accuracy, but also prevent off-pathway aggregation events that can be detrimental to cells. The posttranslational targeting of tail-anchored proteins (TAs) provides tractable model systems to probe these fundamental issues. Recent advances in understanding TA-targeting pathways reveal sophisticated molecular machineries that drive and regulate these processes. These findings also suggest how an interconnected network of targeting factors, cochaperones, and quality control machineries together ensures robust membrane protein biogenesis.

Targeting of tail-anchored membrane proteins to subcellular organelles in Toxoplasma gondii

Proper protein localization is essential for critical cellular processes, including vesicle-mediated transport and protein translocation. Tail-anchored (TA) proteins are integrated into organellar membranes via the C-terminus, orienting the N-terminus towards the cytosol. Localization of TA proteins occurs posttranslationally and is governed by the C-terminus, which contains the integral transmembrane domain (TMD) and targeting sequence. Targeting of TA proteins is dependent on the hydrophobicity of the TMD as well as the length and composition of flanking amino acid sequences. We previously identified an unusual homologue of elongator protein, Elp3, in the apicomplexan parasite Toxoplasma gondii as a TA protein targeting the outer mitochondrial membrane. We sought to gain further insight into TA proteins and their targeting mechanisms using this early-branching eukaryote as a model. Our bioinformatics analysis uncovered 59 predicted TA proteins in Toxoplasma, 9 of which were selected for follow-up analyses based on representative features. We identified novel TA proteins that traffic to specific organelles in Toxoplasma, including the parasite endoplasmic reticulum, mitochondrion, and Golgi apparatus. Domain swap experiments elucidated that targeting of TA proteins to these specific organelles was strongly influenced by the TMD sequence, including charge of the flanking C-terminal sequence.

STRIPAK complexes in cell signaling and cancer

Striatin-interacting phosphatase and kinase (STRIPAK) complexes are striatin-centered multicomponent supramolecular structures containing both kinases and phosphatases. STRIPAK complexes are evolutionarily conserved and have critical roles in protein (de)phosphorylation. Recent studies indicate that STRIPAK complexes are emerging mediators and regulators of multiple vital signaling pathways including Hippo, MAPK (mitogen-activated protein kinase), nuclear receptor and cytoskeleton remodeling. Different types of STRIPAK complexes are extensively involved in a variety of fundamental biological processes ranging from cell growth, differentiation, proliferation and apoptosis to metabolism, immune regulation and tumorigenesis. Growing evidence correlates dysregulation of STRIPAK complexes with human diseases including cancer. In this review, we summarize the current understanding of the assembly and functions of STRIPAK complexes, with a special focus on cell signaling and cancer.

The composition and function of the striatin-interacting phosphatases and kinases (STRIPAK) complex in fungi

The striatin-interacting phosphatases and kinases (STRIPAK) complex is a highly conserved eukaryotic protein complex that was recently described for diverse animal and fungal species. Here, we summarize our current knowledge about the composition and function of the STRIPAK complex from the ascomycete Sordaria macrospora, which we discovered by investigating sexually sterile mutants (pro), having a defect in fruiting body development. Mass spectrometry and yeast two-hybrid analysis defined core subunits of the STRIPAK complex, which have structural homologs in animal and other fungal organisms. These subunits (and their mammalian homologs) are PRO11 (striatin), PRO22 (STRIP1/2), SmMOB3 (Mob3), PRO45 (SLMAP), and PP2AA, the structural, and PP2Ac, the catalytic subunits of protein phosphatase 2A (PP2A). Beside fruiting body formation, the STRIPAK complex controls vegetative growth and hyphal fusion in S. macrospora. Although the contribution of single subunits to diverse cellular and developmental processes is not yet fully understood, functional analysis has already shown that mammalian homologs are able to substitute the function of distinct fungal STRIPAK subunits. This underscores the view that fungal model organisms serve as useful tools to get a molecular insight into cellular and developmental processes of eukaryotes in general. Future work will unravel the precise localization of single subunits within the cell and decipher their STRIPAK-related and STRIPAK-independent functions. Finally, evidence is accumulating that there is a crosstalk between STRIPAK and various signaling pathways, suggesting that eukaryotic development is dependent on STRIPAK signaling.

A fungal sarcolemmal membrane-associated protein (SLMAP) homolog plays a fundamental role in development and localizes to the nuclear envelope, endoplasmic reticulum, and mitochondria

Sarcolemmal membrane-associated protein (SLMAP) is a tail-anchored protein involved in fundamental cellular processes, such as myoblast fusion, cell cycle progression, and chromosomal inheritance. Further, SLMAP misexpression is associated with endothelial dysfunctions in diabetes and cancer. SLMAP is part of the conserved striatin-interacting phosphatase and kinase (STRIPAK) complex required for specific signaling pathways in yeasts, filamentous fungi, insects, and mammals. In filamentous fungi, STRIPAK was initially discovered in Sordaria macrospora, a model system for fungal differentiation. Here, we functionally characterize the STRIPAK subunit PRO45, a homolog of human SLMAP. We show that PRO45 is required for sexual propagation and cell-to-cell fusion and that its forkhead-associated (FHA) domain is essential for these processes. Protein-protein interaction studies revealed that PRO45 binds to STRIPAK subunits PRO11 and SmMOB3, which are also required for sexual propagation. Superresolution structured-illumination microscopy (SIM) further established that PRO45 localizes to the nuclear envelope, endoplasmic reticulum, and mitochondria. SIM also showed that localization to the nuclear envelope requires STRIPAK subunits PRO11 and PRO22, whereas for mitochondria it does not. Taken together, our study provides important insights into fundamental roles of the fungal SLMAP homolog PRO45 and suggests STRIPAK-related and STRIPAK-unrelated functions.

STRIPAK complexes: structure, biological function, and involvement in human diseases

The mammalian striatin family consists of three proteins, striatin, S/G2 nuclear autoantigen, and zinedin. Striatin family members have no intrinsic catalytic activity, but rather function as scaffolding proteins. Remarkably, they organize multiple diverse, large signaling complexes that participate in a variety of cellular processes. Moreover, they appear to be regulatory/targeting subunits for the major eukaryotic serine/threonine protein phosphatase 2A. In addition, striatin family members associate with germinal center kinase III kinases as well as other novel components, earning these assemblies the name striatin-interacting phosphatase and kinase (STRIPAK) complexes. Recently, there has been a great increase in functional and mechanistic studies aimed at identifying and understanding the roles of STRIPAK and STRIPAK-like complexes in cellular processes of multiple organisms. These studies have identified novel STRIPAK and STRIPAK-like complexes and have explored their roles in specific signaling pathways. Together, the results of these studies have sparked increased interest in striatin family complexes because they have revealed roles in signaling, cell cycle control, apoptosis, vesicular trafficking, Golgi assembly, cell polarity, cell migration, neural and vascular development, and cardiac function. Moreover, STRIPAK complexes have been connected to clinical conditions, including cardiac disease, diabetes, autism, and cerebral cavernous malformation. In this review, we discuss the expression, localization, and protein domain structure of striatin family members. Then we consider the diverse complexes these proteins and their homologs form in various organisms, emphasizing what is known regarding function and regulation. Finally, we explore possible roles of striatin family complexes in disease, especially cerebral cavernous malformation.

Tiered assembly of the yeast Far3-7-8-9-10-11 complex at the endoplasmic reticulum

Target of rapamycin signaling is a conserved, essential pathway integrating nutritional cues with cell growth and proliferation. The target of rapamycin kinase exists in two distinct complexes, TORC1 and TORC2. It has been reported that protein phosphatase 2A (PP2A) and the Far3-7-8-9-10-11 complex (Far complex) negatively regulate TORC2 signaling in yeast. The Far complex, originally identified as factors required for pheromone-induced cell cycle arrest, and PP2A form the yeast counterpart of the STRIPAK complex, which was first isolated in mammals. The cellular localization of the Far complex has yet to be fully characterized. Here, we show that the Far complex localizes to the endoplasmic reticulum (ER) by analyzing functional GFP-tagged Far proteins in vivo. We found that Far9 and Far10, two homologous proteins each with a tail-anchor domain, localize to the ER in mutant cells lacking the other Far complex components. Far3, Far7, and Far8 form a subcomplex, which is recruited to the ER by Far9/10. The Far3-7-8- complex in turn recruits Far11 to the ER. Finally, we show that the tail-anchor domain of Far9 is required for its optimal function in TORC2 signaling. Our study reveals tiered assembly of the yeast Far complex at the ER and a function for Far complex's ER localization in TORC2 signaling.

A novel disease gene for Brugada syndrome: sarcolemmal membrane-associated protein gene mutations impair intracellular trafficking of hNav1.5

BACKGROUND

Mutations in genes including SCN5A encoding the α-subunit of the cardiac sodium channel (hNav1.5) cause Brugada syndrome via altered function of cardiac ion channels, but more than two-thirds of Brugada syndrome remains pathogenetically elusive. T-tubules and sarcoplasmic reticulum are essential in excitation of cardiomyocytes, and sarcolemmal membrane-associated protein (SLMAP) is a protein of unknown function localizing at T-tubules and sarcoplasmic reticulum.

METHODS AND RESULTS

We analyzed 190 unrelated Brugada syndrome patients for mutations in SLMAP. Two missense mutations, Val269Ile and Glu710Ala, were found in heterozygous state in 2 patients but were not found in healthy individuals. Membrane surface expression of hNav1.5 in the transfected cells was affected by the mutations, and silencing of mutant SLMAP by small interfering RNA rescued the surface expression of hNav1.5. Whole-cell patch-clamp recordings of hNav1.5-expressing cells transfected with mutant SLMAP confirmed the reduced hNav1.5 current.

CONCLUSIONS

The mutations in SLMAP may cause Brugada syndrome via modulating the intracellular trafficking of hNav1.5 channel.

Tail-anchored membrane protein SLMAP is a novel regulator of cardiac function at the sarcoplasmic reticulum

Sarcolemmal membrane-associated proteins (SLMAPs) are components of cardiac membranes involved in excitation-contraction (E-C) coupling. Here, we assessed the role of SLMAP in cardiac structure and function. We generated transgenic (Tg) mice with cardiac-restricted overexpression of SLMAP1 bearing the transmembrane domain 2 (TM2) to potentially interfere with endogenous SLMAP through homodimerization and subcellular targeting. Histological examination revealed vacuolated myocardium; the severity of which correlated with the expression level of SLMAP1-TM2. High resolution microscopy showed dilation of the sarcoplasmic reticulum/endoplasmic reticulum (SR/ER) and confocal imaging combined with biochemical analysis indicated targeting of SLMAP1-TM2 to the SR/ER membranes and inappropriate homodimerization. Older (28 wk of age) Tg mice exhibited reduced contractility with impaired relaxation as assessed by left ventricle pressure monitoring. The ventricular dysfunction was associated with electrophysiological abnormalities (elongated QT interval). Younger (5 wk of age) Tg mice also exhibited an elongated QT interval with minimal functional disturbances associated with the activation of the fetal gene program. They were less responsive to isoproterenol challenge (ΔdP/dt(max)) and developed electrical and left ventricular pressure alternans. The altered electrophysiological and functional disturbances in Tg mice were associated with diminished expression level of calcium cycling proteins of the sarcoplasmic reticulum such as the ryanodine receptor, Ca(2+)-ATPase, calsequestrin, and triadin (but not phospholamban), as well as significantly reduced calcium uptake in microsomal fractions. These data demonstrate that SLMAP is a regulator of E-C coupling at the level of the SR and its perturbation results in progressive deterioration of cardiac electrophysiology and function.

Contribution of topology determinants of a viral movement protein to its membrane association, intracellular traffic, and viral cell-to-cell movement

The p7B movement protein (MP) of Melon necrotic spot virus (MNSV) is a single-pass membrane protein associated with the endoplasmic reticulum (ER), the Golgi apparatus (GA), and plasmodesmata (Pd). Experimental data presented here revealed that the p7B transmembrane domain (TMD) was sufficient to target the green fluorescent protein (GFP) to ER membranes. In addition, the short extramembrane regions of p7B were essential for subsequent ER export and transport to the GA and Pd. Microsomal partitioning and bimolecular fluorescence assays supported a type II topology of p7B in planta. Mutations affecting conventional determinants of p7B membrane topology, such as the TMD secondary structure, the overall hydrophobicity profile, the so-called "aromatic belt," and the net charge distribution on either side of the TMD, were engineered into infectious RNAs to investigate the relationship between the MP structure and MNSV cell-to-cell movement. The results revealed that (i) the overall hydrophobic profile and the α-helix integrity of the TMD were relevant for virus movement, (ii) modification of the net charge balance of the regions flanking both TMD sides drastically reduced cell-to-cell movement, (iii) localization of p7B to the GA was necessary but not sufficient for virus movement, and (iv) membrane insertion was essential for p7B function in virus movement. Our results therefore indicate that MNSV cell-to-cell movement requires sequential transport of p7B from the ER via the GA to Pd, which is modulated by a combination of several signals with different strengths in the extramembrane regions and TMD of the MP.