Rkp1/Cpc2, a fission yeast RACK1 homolog, is involved in actin cytoskeleton organization through protein kinase C, Pck2, signaling

A WD40 Protein Encoding Gene Fvcpc2 Positively Regulates Mushroom Development and Yield in Flammulina velutipes

Ascomycota and Basidiomycota are two closely related phyla and fungi in two phyla share some common morphological developmental process during fruiting body formation. In Neurospora crassa, the Gβ-like protein CPC-2 with a seven-WD40 repeat domain was previously reported. By transforming CPC-2 ortholog encoding genes, from 7 different fungal species across Ascomycota and Basidiomycota, into the cpc-2 deletion mutant of N. crassa, we demonstrate that all tested CPC-2 ortholog genes were able to complement the defects of the cpc-2 deletion mutant in sexual development, indicating that CPC-2 proteins from Ascomycota and Basidiomycota have the similar cellular function. Using Flammulina velutipes as a model system for mushroom species, the CPC-2 ortholog FvCPC2 was characterized. Fvcpc2 increased transcription during fruiting body development. Knockdown of Fvcpc2 by RNAi completely impaired fruiting body formation. In three Fvcpc2 knockdown mutants, transcriptional levels of genes encoding adenylate cyclase and protein kinase A catalytic subunit were significantly lower and colony growth became slower than wild type. The addition of cAMP or the PKA-activator 8-Bromo-cAMP into the medium restored the Fvcpc2 knockdown mutants to the wild-type colony growth phenotype, suggesting that the involvement of cAMP production in the regulatory mechanisms of FvCPC2. Knockdown of Fvcpc2 also weakened transcriptional responses to sexual development induction by some genes related to fruiting body development, including 4 jacalin-related lectin encoding genes, 4 hydrophobin encoding genes, and 3 functionally-unknown genes, suggesting the participation of these genes in the mechanisms by which FvCPC2 regulates fruiting body development. All three Fvcpc2 overexpression strains displayed increased mushroom yield and shortened cultivation time compared to wild type, suggesting that Fvcpc2 can be a promising reference gene for Winter Mushroom breeding. Since the orthologs of FvCPC2 were highly conserved and specifically expressed during fruiting body development in different edible mushrooms, genes encoding FvCPC2 orthologs in other mushroom species also have potential application in breeding.

RACK1 to the future--a historical perspective

This perspective summarises the first and long overdue RACK1 meeting held at the University of Limerick, Ireland, May 2013, in which RACK1's role in the immune system, the heart and the brain were discussed and its contribution to disease states such as cancer, cardiac hypertrophy and addiction were described. RACK1 is a scaffolding protein and a member of the WD repeat family of proteins. These proteins have a unique architectural assembly that facilitates protein anchoring and the stabilisation of protein activity. A large body of evidence is accumulating which is helping to define the versatile role of RACK1 in assembling and dismantling complex signaling pathways from the cell membrane to the nucleus in health and disease. In this commentary, we first provide a historical perspective on RACK1. We also address many of the pertinent and topical questions about this protein such as its role in transcription, epigenetics and translation, its cytoskeletal contribution and the merits of targeting RACK1 in disease.

'Life is a highway': membrane trafficking during cytokinesis

Cytokinesis, the final stage of the cell cycle, is an essential step toward the formation of two viable daughter cells. In recent years, membrane trafficking has been shown to be important for the completion of cytokinesis. Vesicles originating from both the endocytic and secretory pathways are known to be shuttled to the plasma membrane of the ingressing cleavage furrow, delivering membrane and proteins to this dynamic region. Advances in cell imaging have led to exciting new discoveries regarding vesicle movement in living cells. Recent work has revealed a significant role for membrane trafficking, as controlled by regulatory proteins, during cytokinesis in animal cells. The endocytic and secretory pathways as well as motor proteins are revealed to be essential in the delivery of vesicles to the cleavage furrow during cytokinesis.

A large complex mediated by Moc1, Moc2 and Cpc2 regulates sexual differentiation in fission yeast

Sexual differentiation in Schizosaccharomyces pombe is triggered by nutrient starvation and is downregulated by cAMP. Screening programs have identified the moc1/sds23, moc2/ded1, moc3 and moc4/zfs1 genes as inducers of sexual differentiation, even in the presence of elevated levels of cAMP. To investigate possible interactions among Moc1, Moc2, Moc3 and Moc4 proteins, we first screened for individual Moc-interacting proteins using the yeast two-hybrid system and verified the interactions with other Moc proteins. Using this screening process, Cpc2 and Rpl32-2 were highlighted as factors involved in interactions with multiple Moc proteins. Cpc2 interacted with Moc1, Moc2 and Moc3, whereas the ribosomal protein Rpl32-2 interacted with all Moc proteins in the two-hybrid system. Physical interactions of Cpc2 with Moc1, Moc2 and Rpl32-2, and of Rpl32-2 with Moc2 were confirmed by coimmunoprecipitation. In addition, using Blue Native/PAGE, we revealed that each Moc protein exists as a large complex. Overexpression of Moc1, Moc2, Moc3, Moc4 and Rpl32-2 resulted in the efficient induction of a key transcription factor Ste11, suggesting that all proteins tested are positive regulators of Ste11. Considering that Moc2/Ded1 is a general translation factor and that Cpc2 associates with many ribosomal proteins, including Rpl32-2, it is possible that a large Moc-mediated complex, detected in this study, may act as a translational regulator involved in the control of sexual differentiation in S. pombe through the induction of Ste11.

Role for RACK1 orthologue Cpc2 in the modulation of stress response in fission yeast

The receptor of activated C kinase (RACK1) is a protein highly conserved among eukaryotes. In mammalian cells, RACK1 functions as an adaptor to favor protein kinase C (PKC)-mediated phosphorylation and subsequent activation of c-Jun NH(2)-terminal kinase mitogen-activated protein kinase. Cpc2, the RACK1 orthologue in the fission yeast Schizosaccharomyces pombe, is involved in the control of G2/M transition and interacts with Pck2, a PKC-type protein member of the cell integrity Pmk1 mitogen-activated protein kinase (MAPK) pathway. Both RACK1 and Cpc2 are structural components of the 40S ribosomal subunit, and recent data suggest that they might be involved in the control of translation. In this work, we present data supporting that Cpc2 negatively regulates the cell integrity transduction pathway by favoring translation of the tyrosine-phosphatases Pyp1 and Pyp2 that deactivate Pmk1. In addition, Cpc2 positively regulates the synthesis of the stress-responsive transcription factor Atf1 and the cytoplasmic catalase, a detoxificant enzyme induced by treatment with hydrogen peroxide. These results provide for the first time strong evidence that the RACK1-type Cpc2 protein controls from the ribosome the extent of the activation of MAPK cascades, the cellular defense against oxidative stress, and the progression of the cell cycle by regulating positively the translation of specific gene products involved in key biological processes.

RACK-1 directs dynactin-dependent RAB-11 endosomal recycling during mitosis in Caenorhabditis elegans

Membrane trafficking pathways are necessary for the addition and removal of membrane during cytokinesis. In animal cells, recycling endosomes act as a major source of the additional membranes during furrow progression and abscission. However, the mechanisms and factors that regulate recycling endosomes during the cell cycle remain poorly understood. Here, we show that the Caenorhabditis elegans Receptor of Activated C Kinase 1 (RACK-1) is required for cytokinesis, germline membrane organization, and the recruitment of RAB-11-labeled recycling endosomes to the pericentrosomal region and spindle. RACK-1 is also required for proper chromosome separation and astral microtubule length. RACK-1 localizes to the centrosomes, kinetochores, the midbody, and nuclear envelopes during the cell cycle. We found that RACK-1 directly binds to DNC-2, the C. elegans p50/dynamitin subunit of the dynactin complex. Last, RACK-1 may facilitate the sequestration of recycling endosomes by targeting DNC-2 to centrosomes and the spindle. Our findings suggest a mechanism by which RACK-1 directs the dynactin-dependent redistribution of recycling endosomes during the cell cycle, thus ensuring proper membrane trafficking events during cytokinesis.

Rapid screen of human genes for relevance to cancer using fission yeast

A total of 437 human full-length cDNAs isolated by microarray analysis of liver and/or gastric cancer tissues were evaluated for their relevance to cancer using the fission yeast Schizosaccharomyces pombe. Overexpression of 161 human cDNAs in S. pombe caused growth inhibition and/or morphological changes, which can be considered as cancer-related phenotypes of S. pombe. Sixteen genes causing growth defects and morphological changes at the same time were chosen to validate their ostensible oncogenic properties. They were highly expressed in liver and/or gastric cancer cell lines. Also, when the mouse embryonic fibroblast cell type NIH3T3 was transfected with these genes, the proliferation rates of cells were increased by 32% to 120%. This study demonstrates that fission yeast can be used as an advantageous and powerful tool for the rapid screening of human genes relevant to cancer. Furthermore, the human genes screened can be tested further as diagnostic markers and potential therapeutic targets for liver and stomach cancers. They also can be studied further for the elucidation of mechanisms involved in carcinogenesis.

The RACK1 homologue from Trypanosoma brucei is required for the onset and progression of cytokinesis

The receptor for activated C kinase 1 (RACK1) is a conserved scaffold protein that helps regulate a range of cell activities including cell growth, shape, and protein translation. We report that a homologue of RACK1 is required for cytokinesis in pathogenic Trypanosoma brucei. The protein, referred to as TRACK, is comprised of WD repeat elements and can complement cpc2 null mutants of Schizosaccharomyces pombe. TRACK is expressed throughout the trypanosome life cycle and is distributed predominantly in a perinuclear region and the cytoplasm but not along the endoplasmic reticulum, mitochondrion, or cleavage furrow of dividing cells. When tetracycline-inducible RNA interference (RNAi) is used to deplete the cellular content of TRACK, the cells remain metabolically active, but growth is inhibited. In bloodstream forms, growth arrest is due to a delay in the onset of cytokinesis. By contrast, procyclic forms are able to initiate cytokinesis in the absence of TRACK but arrest midway through cell cleavage. The RNAi cells undergo multiple rounds of partial cytokinesis and accumulate nuclei and cytoplasmic extensions with attached flagella. The TRACK RNAi construct is also inducible within infected mice. Under these conditions parasites are eliminated from peripheral blood within 3 days post-infection. Taken as a whole, these data indicate that trypanosomes utilize a RACK1 homologue to regulate the final stages of mitosis. Moreover, disrupting the interaction between TRACK and its partners might be targeted in the design of novel therapies.

The yeast CPC2/ASC1 gene is regulated by the transcription factors Fhl1p and Ifh1p

CPC2/ASC1 is one of the most abundantly transcribed genes in Saccharomyces cerevisiae. It encodes a ribosome-associated Gbeta-like WD protein, which is highly conserved from yeast to man. Here, we show that CPC2 transcription depends on the carbon source and is induced during utilization of the sugar glucose. CPC2 promoter deletion and insertion analyses identified two upstream activation sequence elements for CPC2, which are required for basal expression and regulation. One of these upstream activation sequence elements has an ATGTACGGATGT motif, which has previously been described as a putative binding site for the forkhead-like transcription factor Fhl1p. Deletion of FHL1 reduces CPC2 transcription significantly in presence of glucose, but has no effect when the non-fermentable carbon source ethanol is provided. Increased amounts of the Fhl1p co-regulator Ifh1p induce CPC2 transcription even when ethanol is utilized. These data suggest that the interaction between Fhl1p and Ifh1p is critical for the regulation of CPC2 transcription during utilization of different carbon sources.

Dissection of the mammalian midbody proteome reveals conserved cytokinesis mechanisms

Cytokinesis is the essential process that partitions cellular contents into daughter cells. To identify and characterize cytokinesis proteins rapidly, we used a functional proteomic and comparative genomic strategy. Midbodies were isolated from mammalian cells, proteins were identified by multidimensional protein identification technology (MudPIT), and protein function was assessed in Caenorhabditis elegans. Of 172 homologs disrupted by RNA interference, 58% displayed defects in cleavage furrow formation or completion, or germline cytokinesis. Functional dissection of the midbody demonstrated the importance of lipid rafts and vesicle trafficking pathways in cytokinesis, and the utilization of common membrane cytoskeletal components in diverse morphogenetic events in the cleavage furrow, the germline, and neurons.

Human malaria parasites display a receptor for activated C kinase ortholog

Receptors for activated C kinases (RACKs) are scaffold proteins that anchor diverse signaling proteins and are involved in modulating cell cycle. We report the cloning and cellular localization of a RACK ortholog (PfRACK) in the human malaria parasite Plasmodium falciparum. The full-length transcript obtained by 3(') and 5(') RACE has 1.4 kbp with a predicted ORF of 972 bp, coding for a protein with 323 residues of 35.8 kDa molecular weight and pI 6.38. PfRACK has 59% and 60% identity at the amino acid level to Chlamydomonas reinhardtii and Danio rerio RACKs, respectively, presenting seven WD40 motifs and retaining the conserved domains in repeats III (DVFSVSF) and VI (STINSLCF) that are important for PKC binding. Semi-quantitative RT-PCR revealed that PfRACK is constitutively expressed in the intraerythrocytic stages of P. falciparum. Using confocal microscopy, PfRACK was immunolocalized in all parasite stages, being conspicuously spread throughout the schizont. The high similarity of PfRACK to those previously described in other organisms, as well as its constitutive expression in Plasmodium asexual stages, suggests that it might play a key role in the regulatory processes of malaria parasite life cycle.

Cloning, expression and genomic structure of a novel human GNB2L1 gene, which encodes a receptor of activated protein kinase C (RACK)

During a large-scale screen of a human fetal brain cDNA library, a novel human gene GNB2L1 encoding a novel RACK (receptor of activated protein kinase C) protein was isolated and sequenced. The cDNA is 1142 bp long and has a predicted open reading frame encoding 316 aa. The predicted protein shows higher similarity to rat RACK1 and many RACK proteins of different organisms including Drosophila, C. elegans, mouse, rat, human, C. fasciculata, zebrafish, A. thaliana, S. cerevisiae and so on, suggesting it is conserved during evolution. The gene was mapped to human chromosome 5q35.3, the telomer position of chromosome 5q, in which the disease gene for early-onset primary congenital lymphedema was mapped. Also, 5q35.3 is a frequently reported location for cytogenetic and molecular abnormalities in renal cell carcinomas. The gene has 8 exons and 7 introns. It is expressed ubiquitously in many human tissues detected by northern blot analysis and RT-PCR.

Pleckstrin homology domain interacts with Rkp1/Cpc2, a RACK1 homolog, to modulate Pck2-mediated signaling process in Schizosaccharomyces pombe

Rkp1/Cpc2, a fission yeast RACK1 homolog, interacts with Pck2, a PKC homolog, and is involved in the regulation of pck2-mediated signaling process. The N-terminal region of split pleckstrin homology domain (nPH) in human PLC-gamma1 bound to Rkp1/Cpc2 concomitantly with Pck2. nPH inhibited kinase activity of GST-Pck2 purified from Schizosaccharomyces pombe in vitro. The lethality induced by pck2(+) overexpression was suppressed by coexpression of either rkp1(+) or nPH domain. This result suggests that Rkp1/Cpc2 interacts with PH domain-containing protein and regulates the Pck2-mediated signaling process in S. pombe.