Structural evidence for variable oligomerization of the N-terminal domain of cyclase-associated protein (CAP)

Mechanisms of actin disassembly and turnover

Cellular actin networks exhibit a wide range of sizes, shapes, and architectures tailored to their biological roles. Once assembled, these filamentous networks are either maintained in a state of polarized turnover or induced to undergo net disassembly. Further, the rates at which the networks are turned over and/or dismantled can vary greatly, from seconds to minutes to hours or even days. Here, we review the molecular machinery and mechanisms employed in cells to drive the disassembly and turnover of actin networks. In particular, we highlight recent discoveries showing that specific combinations of conserved actin disassembly-promoting proteins (cofilin, GMF, twinfilin, Srv2/CAP, coronin, AIP1, capping protein, and profilin) work in concert to debranch, sever, cap, and depolymerize actin filaments, and to recharge actin monomers for new rounds of assembly.

Native cyclase-associated protein and actin from Xenopus laevis oocytes form a unique 4:4 complex with a tripartite structure

Cyclase-associated protein (CAP) is a conserved actin-binding protein that regulates multiple aspects of actin dynamics, including polymerization, depolymerization, filament severing, and nucleotide exchange. CAP has been isolated from different cells and tissues in an equimolar complex with actin, and previous studies have shown that a CAP–actin complex contains six molecules each of CAP and actin. Intriguingly, here, we successfully isolated a complex of Xenopus cyclase-associated protein 1 (XCAP1) with actin from oocyte extracts, which contained only four molecules each of XCAP1 and actin. This XCAP1–actin complex remained stable as a single population of 340 kDa species during hydrodynamic analyses using gel filtration or analytical ultracentrifugation. Examination of the XCAP1–actin complex by high-speed atomic force microscopy revealed a tripartite structure: one middle globular domain and two globular arms. The two arms were observed in high and low states. The arms at the high state were spontaneously converted to the low state by dissociation of actin from the complex. However, when extra G-actin was added, the arms at the low state were converted to the high state. Based on the known structures of the N-terminal helical-folded domain and C-terminal CARP domain, we hypothesize that the middle globular domain corresponds to a tetramer of the N-terminal helical-folded domain of XCAP1 and that each arm in the high state corresponds to a heterotetramer containing a dimer of the C-terminal CARP domain of XCAP1 and two G-actin molecules. This novel configuration of a CAP–actin complex should help to understand how CAP promotes actin filament disassembly.

CAPt'n of Actin Dynamics: Recent Advances in the Molecular, Developmental and Physiological Functions of Cyclase-Associated Protein (CAP)

Cyclase-associated protein (CAP) has been discovered three decades ago in budding yeast as a protein that associates with the cyclic adenosine monophosphate (cAMP)-producing adenylyl cyclase and that suppresses a hyperactive RAS2 variant. Since that time, CAP has been identified in all eukaryotic species examined and it became evident that the activity in RAS-cAMP signaling is restricted to a limited number of species. Instead, its actin binding activity is conserved among eukaryotes and actin cytoskeleton regulation emerged as its primary function. However, for many years, the molecular functions as well as the developmental and physiological relevance of CAP remained unknown. In the present article, we will compile important recent progress on its molecular functions that identified CAP as a novel key regulator of actin dynamics, i.e., the spatiotemporally controlled assembly and disassembly of actin filaments (F-actin). These studies unraveled a cooperation with ADF/Cofilin and Twinfilin in F-actin disassembly, a nucleotide exchange activity on globular actin monomers (G-actin) that is required for F-actin assembly and an inhibitory function towards the F-actin assembly factor INF2. Moreover, by focusing on selected model organisms, we will review current literature on its developmental and physiological functions, and we will present studies implicating CAP in human pathologies. Together, this review article summarizes and discusses recent achievements in understanding the molecular, developmental and physiological functions of CAP, which led this protein emerge as a novel CAPt'n of actin dynamics.

Cyclase-associated protein 2 dimerization regulates cofilin in synaptic plasticity and Alzheimer's disease

Regulation of actin cytoskeleton dynamics in dendritic spines is crucial for learning and memory formation. Hence, defects in the actin cytoskeleton pathways are a biological trait of several brain diseases, including Alzheimer's disease. Here, we describe a novel synaptic mechanism governed by the cyclase-associated protein 2, which is required for structural plasticity phenomena and completely disrupted in Alzheimer's disease. We report that the formation of cyclase-associated protein 2 dimers through its Cys³² is important for cyclase-associated protein 2 binding to cofilin and for actin turnover. The Cys³²-dependent cyclase-associated protein 2 homodimerization and association to cofilin are triggered by long-term potentiation and are required for long-term potentiation-induced cofilin translocation into spines, spine remodelling and the potentiation of synaptic transmission. This mechanism is specifically affected in the hippocampus, but not in the superior frontal gyrus, of both Alzheimer's disease patients and APP/PS1 mice, where cyclase-associated protein 2 is down-regulated and cyclase-associated protein 2 dimer synaptic levels are reduced. Notably, cyclase-associated protein 2 levels in the cerebrospinal fluid are significantly increased in Alzheimer's disease patients but not in subjects affected by frontotemporal dementia. In Alzheimer's disease hippocampi, cofilin association to cyclase-associated protein 2 dimer/monomer is altered and cofilin is aberrantly localized in spines. Taken together, these results provide novel insights into structural plasticity mechanisms that are defective in Alzheimer's disease.

Cyclase-associated protein 1 is a key negative regulator of milk synthesis and proliferation of bovine mammary epithelial cells

Adenylyl cyclase-associated protein (CAP) is a highly conserved protein. Previous reports have suggested that CAP1 may be a negative regulator of cellular proliferation, migration, and adhesion and the development of cell carcinomas. The molecular mechanism of CAP1 regulation of downstream pathways, as well as how CAP1 is regulated by environmental stimuli and upstream signalling, is not well understood. In this present study, we assessed the role of CAP1 in milk synthesis and proliferation of bovine mammary epithelial cells. Using gene overexpression and silencing methods, CAP1 was found to negatively regulate milk synthesis and proliferation of cells via the PI3K-mTOR/SREBP-1c/Cyclin D1 signalling pathway. Hormones, such as prolactin and oestrogen, and amino acids, such as methionine and leucine, stimulate MMP9 expression and trigger CAP1 degradation, and thus, abrogate its inhibition of synthesis of milk protein, fat, and lactose by and proliferation of bovine mammary epithelial cells. The results of our study help deepen our understanding of the regulatory mechanisms underlying milk synthesis and aid in characterizing the molecular mechanisms of CAP1. Previous reports have suggested that CAP1 is a negative regulator of cellular proliferation and anabolism, but the molecular mechanisms are largely unknown. In this present study, we identified CAP1 as a negative regulator of milk synthesis and proliferation of bovine mammary epithelial cells. Our results will deepen our understanding of the regulatory mechanisms underlying milk synthesis and aid in exploring the molecular mechanisms of CAP1.

CAP2 mutation leads to impaired actin dynamics and associates with supraventricular tachycardia and dilated cardiomyopathy

Background

Dilated cardiomyopathy (DCM) is a primary myocardial disease leading to contractile dysfunction, progressive heart failure and excessive risk of sudden cardiac death. Around half of DCM cases are idiopathic, and genetic factors seem to play an important role.

Aim

We investigated a possible genetic cause of DCM in two consanguineous children from a Bedouin family.

Methods and results

Using exome sequencing and searching for rare homozygous variations, we identified a nucleotide change in the donor splice consensus sequence of exon 7 in CAP2 as the causative mutation. Using patient-derived fibroblasts, we demonstrated that the mutation causes skipping of exons 6 and 7. The resulting protein is missing 64 amino acids in its N-CAP domain that should prevent its correct folding. CAP2 protein level was markedly reduced without notable compensation by the homolog CAP1. However, β-actin mRNA was elevated as demonstrated by real-time qPCR. In agreement with the essential role of CAP2 in actin filament polymerization, we demonstrate that the mutation affects the kinetics of repolymerization of actin in patient fibroblasts.

Conclusions

This is the first report of a recessive deleterious mutation in CAP2 and its association with DCM in humans. The clinical phenotype recapitulates the damaging effects on the heart observed in Cap2 knockout mice including DCM and cardiac conduction disease, but not the other effects on growth, viability, wound healing and eye development. Our data underscore the importance of the proper kinetics of actin polymerization for normal function of the human heart.

Conserved hydrophobic residues in the CARP/β-sheet domain of cyclase-associated protein are involved in actin monomer regulation

Cyclase-associated protein (CAP) is a multidomain protein that promotes actin filament dynamics. The C-terminal region of CAP contains a CAP and X-linked retinitis pigmentosa 2 protein (CARP) domain (or a β-sheet domain), which binds to actin monomer and is essential for enhancing exchange of actin-bound nucleotides. However, how the CARP domain binds to actin is not clearly understood. Here, we report that conserved hydrophobic residues in the CARP domain play important roles in the function of CAP to regulate actin dynamics. Single mutations of three conserved surface-exposed hydrophobic residues in the CARP domain of CAS-2, a Caenorhabditis elegans CAP, significantly reduce its binding to actin monomers and suppress its nucleotide exchange activity on actin. As a result, these mutants are weaker than wild-type to compete with ADF/cofilin to promote recycling of actin monomers for polymerization. A double mutation (V367A/I373A) eliminates these actin-regulatory functions of CAS-2. These hydrophobic residues and previously identified functional residues are scattered on a concave β-sheet of the CARP domain, suggesting that a wide area of the β-sheet is involved in binding to actin. These observations suggest that the CARP domain of CAP binds to actin in a distinct manner from other known actin-binding proteins.

CAP1 is overexpressed in human epithelial ovarian cancer and promotes cell proliferation

Adenylate cyclase-associated protein 1 (CAP1) regulates both actin filaments and the Ras/cAMP pathway in yeast, and has been found play a role in cell motility and in the development of certain types of cancer. In the present study, we investigated CAP1 gene expression in human epithelial ovarian cancer (EOC). Western blot analysis and immunohistochemistry were performed using EOC tissue samples and the results revealed that CAP1 expression increased with the increasing grade of EOC. In the normal ovarian tissue samples however, CAP1 expression was barely detected. Using Pearson’s χ² test, it was demonstrated that CAP1 expression was associated with the histological grade and Ki-67 expression. Kaplan-Meier analysis revealed that a higher CAP1 expression in patients with EOC was associated with a poorer prognosis. In in vitro experiments using HO-8910 EOC cells, the expression of CAP1 was knocked down using siRNA. The proliferation of the HO-8910 cells was then determined by cell cycle analysis and cell proliferation assay using the cell counting kit-8 and flow cytometry. The results revealed that the loss of CAP1 expression inhibited cell cycle progression. These findings suggest that a high expression of CAP1 is involved in the pathogenesis of EOC, and that the downregulation of CAP1 in tumor cells may be a therapeutic target for the treatment of patients with EOC.

Relationship between expression of matrix metalloproteinase-9 and adenylyl cyclase-associated protein 1 in chronic obstructive pulmonary disease

OBJECTIVE

To investigate the relationship between expression of matrix metalloproteinase (MMP)-9 and expression of adenylyl cyclase-associated protein (CAP)-1 in chronic obstructive pulmonary disease (COPD).

METHODS

Patients with possible respiratory disease were recruited into the study and divided into a COPD group and a non-COPD group on diagnosis. Pulmonary function tests were performed and serum concentrations of MMP-9 were measured using an enzyme-linked immunosorbent assay. MMP-9 and CAP1 expression were analysed in lung tissue and bronchoalveolar lavage fluid in all available samples using immunohistochemistry and Western blot, respectively. In addition, expression of MMP-9 and CAP1 in vitro was investigated using immunofluorescence. Expression of CAP1 in response to MMP-9 was measured in the human alveolar epithelial cell line HP-AEpiC, using Western blot.

RESULTS

A total of 90 patients were included in the study: 52 were in the COPD group and 38 in the non-COPD group. Serum MMP-9 concentrations were significantly higher in the COPD than in the non-COPD group. MMP-9 serum concentrations were negatively correlated with forced expiratory volume in 1 s (FEV1), FEV1 as a percentage of the normal predicted value and the ratio of FEV1 to forced vital capacity, and were positively correlated with residual volume (RV), total lung capacity (TLC) and RV/TLC values. In lung tissue and bronchoalveolar lavage fluid samples, MMP-9 and CAP1 expression were inversely related. This relationship was confirmed in HP-AEpiC cells. High expression of MMP-9 and low expression of CAP1 was demonstrated in the COPD group compared with the non-COPD group.

CONCLUSIONS

This study demonstrated an inverse relationship between CAP1 and MMP-9 expression, and high expression of MMP-9 and low expression of CAP1 in those with COPD compared with the non-COPD group. Overexpression of MMP-9 in lung tissue and its interaction with CAP1 is likely to play a major role in airway obstruction in COPD.

Phosphorylation of the cytoskeletal protein CAP1 controls its association with cofilin and actin

Cell signaling can control the dynamic balance between filamentous and monomeric actin by modulating actin regulatory proteins. One family of actin regulating proteins that controls actin dynamics comprises cyclase-associated proteins 1 and 2 (CAP1 and 2, respectively). However, cell signals that regulate CAPs remained unknown. We mapped phosphorylation sites on mouse CAP1 and found S307 and S309 to be regulatory sites. We further identified glycogen synthase kinase 3 as a kinase phosphorylating S309. The phosphomimetic mutant S307D/S309D lost binding to its partner cofilin and, when expressed in cells, caused accumulation of actin stress fibers similar to that in cells with reduced CAP expression. In contrast, the non-phosphorylatable S307A/S309A mutant showed drastically increased cofilin binding and reduced binding to actin. These results suggest that the phosphorylation serves to facilitate release of cofilin for a subsequent cycle of actin filament severing. Moreover, our results suggest that S307 and S309 function in tandem; neither the alterations in binding cofilin and/or actin, nor the defects in rescuing the phenotype of the enlarged cell size in CAP1 knockdown cells was observed in point mutants of either S307 or S309. In summary, we identify a novel regulatory mechanism of CAP1 through phosphorylation.

The role of cyclase-associated protein in regulating actin filament dynamics - more than a monomer-sequestration factor

Dynamic reorganization of the actin cytoskeleton is fundamental to a number of cell biological events. A variety of actin-regulatory proteins modulate polymerization and depolymerization of actin and contribute to actin cytoskeletal reorganization. Cyclase-associated protein (CAP) is a conserved actin-monomer-binding protein that has been studied for over 20 years. Early studies have shown that CAP sequesters actin monomers; recent studies, however, have revealed more active roles of CAP in actin filament dynamics. CAP enhances the recharging of actin monomers with ATP antagonistically to ADF/cofilin, and also promotes the severing of actin filaments in cooperation with ADF/cofilin. Self-oligomerization and binding to other proteins regulate activities and localization of CAP. CAP has crucial roles in cell signaling, development, vesicle trafficking, cell migration and muscle sarcomere assembly. This Commentary discusses the recent advances in our understanding of the functions of CAP and its implications as an important regulator of actin cytoskeletal dynamics, which are involved in various cellular activities.

Autonomous and in trans functions for the two halves of Srv2/CAP in promoting actin turnover

Recent evidence has suggested that Srv2/CAP (cyclase-associated protein) has two distinct functional roles in regulating actin turnover, with its N-terminus enhancing cofilin-mediated severing of actin filaments and its C-terminus catalyzing actin monomer recycling. However, it has remained unclear to what degree these two activities are coordinated by being linked in one molecule, or whether they can function autonomously. To address this, we physically divided the protein into two separate halves, N-Srv2 and C-Srv2, and asked whether they are able to function in trans both in living cells and in reconstituted assays for F-actin turnover and actin-based motility. Remarkably, in F-actin turnover assays the stimulatory effects of N-Srv2 and C-Srv2 functioning in trans were quantitatively similar to those of intact full-length Srv2. Further, in bead motility assays and in vivo, the fragments again functioned in trans, although not with the full effectiveness of intact Srv2. From these data, we conclude that the functions of the two halves of Srv2/CAP are largely autonomous, although their linkage improves coordination of the two functions in specific settings, possibly explaining why the linkage is conserved across distant plant, animal, and fungal species.

Downregulated expression of the cyclase-associated protein 1 (CAP1) reduces migration in esophageal squamous cell carcinoma

OBJECTIVE

Overexpression of cyclase-associated proteins has been associated with poor prognosis in several human cancers. Cyclase-associated protein 1 is a member of the cyclase-associated proteins which contributes to tumor progression. The aim of the present study was to examine the expression of cyclase-associated protein 1 and to elucidate its clinicopathologic significance in a larger series of esophageal squamous cell carcinoma.

METHODS

Immunohistochemical and western blot analyses were performed in esophageal squamous cell carcinoma tissues. Survival analyses were performed by using the Kaplan-Meier method. The role of cyclase-associated protein 1 in migration was studied in esophageal squamous cell carcinoma cell lines of TE1 through knocking down cyclase-associated protein 1 with siRNA and overexpression of cyclase-associated protein 1. The regulation of cyclase-associated protein 1 on migration was determined by transwell and wound-healing assays.

RESULTS

Immunohistochemical analysis showed that cyclase-associated protein 1 expression was negatively associated with E-cadherin and significantly associated with lymph node metastases. Survival analysis revealed that cyclase-associated protein 1 overexpression was significantly associated with overall survival (P = 0.011). Knock down of cyclase-associated protein 1 in TE1 cells resulted in decreased vimentin and F-actin levels and the capability for migration. In addition, overexpression of cyclase-associated protein 1 promoted the migration of TE1 cells.

CONCLUSIONS

These findings suggest that cyclase-associated protein 1 is involved in the metastasis of esophageal squamous cell carcinoma, and that elevated levels of cyclase-associated protein 1 expression may indicate a poor prognosis for patients with esophageal squamous cell carcinoma.

Mammalian and malaria parasite cyclase-associated proteins catalyze nucleotide exchange on G-actin through a conserved mechanism

Cyclase-associated proteins (CAPs) are among the most highly conserved regulators of actin dynamics, being present in organisms from mammals to apicomplexan parasites. Yeast, plant, and mammalian CAPs are large multidomain proteins, which catalyze nucleotide exchange on actin monomers from ADP to ATP and recycle actin monomers from actin-depolymerizing factor (ADF)/cofilin for new rounds of filament assembly. However, the mechanism by which CAPs promote nucleotide exchange is not known. Furthermore, how apicomplexan CAPs, which lack many domains present in yeast and mammalian CAPs, contribute to actin dynamics is not understood. We show that, like yeast Srv2/CAP, mouse CAP1 interacts with ADF/cofilin and ADP-G-actin through its N-terminal α-helical and C-terminal β-strand domains, respectively. However, in the variation to yeast Srv2/CAP, mouse CAP1 has two adjacent profilin-binding sites, and it interacts with ATP-actin monomers with high affinity through its WH2 domain. Importantly, we revealed that the C-terminal β-sheet domain of mouse CAP1 is essential and sufficient for catalyzing nucleotide exchange on actin monomers, although the adjacent WH2 domain is not required for this function. Supporting these data, we show that the malaria parasite Plasmodium falciparum CAP, which is entirely composed of the β-sheet domain, efficiently promotes nucleotide exchange on actin monomers. Collectively, this study provides evidence that catalyzing nucleotide exchange on actin monomers via the β-sheet domain is the most highly conserved function of CAPs from mammals to apicomplexan parasites. Other functions, including interactions with profilin and ADF/cofilin, evolved in more complex organisms to adjust the specific role of CAPs in actin dynamics.

Srv2/cyclase-associated protein forms hexameric shurikens that directly catalyze actin filament severing by cofilin

Dual-color total internal reflection fluorescence microscopy revealed that the N-terminal half of Srv2 (N-Srv2) directly catalyzes severing of cofilin-decorated actin filaments. N-Srv2 formed novel six-bladed structures resembling ninja throwing stars (shurikens), and N-Srv2 activities were critical for actin organization in vivo and were lethal in combination with Aip1.

Actin filament severing is critical for the dynamic turnover of cellular actin networks. Cofilin severs filaments, but additional factors may be required to increase severing efficiency in vivo. Srv2/cyclase-associated protein (CAP) is a widely expressed protein with a role in binding and recycling actin monomers ascribed to domains in its C-terminus (C-Srv2). In this paper, we report a new biochemical and cellular function for Srv2/CAP in directly catalyzing cofilin-mediated severing of filaments. This function is mediated by its N-terminal half (N-Srv2), and is physically and genetically separable from C-Srv2 activities. Using dual-color total internal reflection fluorescence microscopy, we determined that N-Srv2 stimulates filament disassembly by increasing the frequency of cofilin-mediated severing without affecting cofilin binding to filaments. Structural analysis shows that N-Srv2 forms novel hexameric star-shaped structures, and disrupting oligomerization impairs N-Srv2 activities and in vivo function. Further, genetic analysis shows that the combined activities of N-Srv2 and Aip1 are essential in vivo. These observations define a novel mechanism by which the combined activities of cofilin and Srv2/CAP lead to enhanced filament severing and support an emerging view that actin disassembly is controlled not by cofilin alone, but by a more complex set of factors working in concert.

CAS-1, a C. elegans cyclase-associated protein, is required for sarcomeric actin assembly in striated muscle

Assembly of contractile apparatuses in striated muscle requires precisely regulated reorganization of the actin cytoskeletal proteins into sarcomeric organization. Regulation of actin filament dynamics is one of the essential processes of myofibril assembly, but the mechanism of actin regulation in striated muscle is not clearly understood. Actin depolymerizing factor (ADF)/cofilin is a key enhancer of actin filament dynamics in striated muscle in both vertebrates and nematodes. Here, we report that CAS-1, a cyclase-associated protein in Caenorhabditis elegans, promotes ADF/cofilin-dependent actin filament turnover in vitro and is required for sarcomeric actin organization in striated muscle. CAS-1 is predominantly expressed in striated muscle from embryos to adults. In vitro, CAS-1 binds to actin monomers and enhances exchange of actin-bound ATP/ADP even in the presence of UNC-60B, a muscle-specific ADF/cofilin that inhibits the nucleotide exchange. As a result, CAS-1 and UNC-60B cooperatively enhance actin filament turnover. The two proteins also cooperate to shorten actin filaments. A cas-1 mutation is homozygous lethal with defects in sarcomeric actin organization. cas-1-mutant embryos and worms have aggregates of actin in muscle cells, and UNC-60B is mislocalized to the aggregates. These results provide genetic and biochemical evidence that cyclase-associated protein is a critical regulator of sarcomeric actin organization in striated muscle.

A central role for the WH2 domain of Srv2/CAP in recharging actin monomers to drive actin turnover in vitro and in vivo

Cellular processes propelled by actin polymerization require rapid disassembly of filaments, and then efficient recycling of ADF/cofilin-bound ADP-actin monomers back to an assembly-competent ATP-bound state. How monomer recharging is regulated in vivo is still not well understood, but recent work suggests the involvement of the ubiquitous actin-monomer binding protein Srv2/CAP. To better understand Srv2/CAP mechanism, we explored the contribution of its WH2 domain, the function of which has remained highly elusive. We found that the WH2 domain binds to actin monomers and, unlike most other WH2 domains, exhibits similar binding affinity for ATP-actin and ADP-actin (K(d) approximately 1.5 microM). Mutations in the WH2 domain that impair actin binding disrupt the ability of purified full-length Srv2/CAP to catalyze nucleotide exchange on ADF/cofilin-bound actin monomers and accelerate actin turnover in vitro. The same mutations impair Srv2/CAP function in vivo in regulating actin organization, cell growth, and cell morphogenesis. Thus, normal cell growth and organization depend on the ability of Srv2/CAP to recharge actin monomers, and the WH2 domain plays a central role in this process. Our data also reveal that while most isolated WH2 domains inhibit nucleotide exchange on actin, WH2 domains in the context of intact proteins can help promote nucleotide exchange.

Structure and function of a G-actin sequestering protein with a vital role in malaria oocyst development inside the mosquito vector

Cyclase-associated proteins (CAPs) are evolutionary conserved G-actin-binding proteins that regulate microfilament turnover. CAPs have a modular structure consisting of an N-terminal adenylate cyclase binding domain, a central proline-rich segment, and a C-terminal actin binding domain. Protozoan parasites of the phylum Apicomplexa, such as Cryptosporidium and the malaria parasite Plasmodium, express small CAP orthologs with homology to the C-terminal actin binding domain (C-CAP). Here, we demonstrate by reverse genetics that C-CAP is dispensable for the pathogenic Plasmodium blood stages. However, c-cap(-) parasites display a complete defect in oocyst development in the insect vector. By trans-species complementation we show that the Cryptosporidium parvum ortholog complements the Plasmodium gene functions. Purified recombinant C. parvum C-CAP protein binds actin monomers and prevents actin polymerization. The crystal structure of C. parvum C-CAP shows two monomers with a right-handed beta-helical fold intercalated at their C termini to form the putative physiological dimer. Our results reveal a specific vital role for an apicomplexan G-actin-binding protein during sporogony, the parasite replication phase that precedes formation of malaria transmission stages. This study also exemplifies how Plasmodium reverse genetics combined with biochemical and structural analyses of orthologous proteins can offer a fast track toward systematic gene characterization in apicomplexan parasites.

Reconstitution and dissection of the 600-kDa Srv2/CAP complex: roles for oligomerization and cofilin-actin binding in driving actin turnover

Srv2/cyclase-associated protein is expressed in virtually all plant, animal, and fungal organisms and has a conserved role in promoting actin depolymerizing factor/cofilin-mediated actin turnover. This is achieved by the abilities of Srv2 to recycle cofilin from ADP-actin monomers and to promote nucleotide exchange (ATP for ADP) on actin monomers. Despite this important and universal role in facilitating actin turnover, the mechanism underlying Srv2 function has remained elusive. Previous studies have demonstrated a critical functional role for the G-actin-binding C-terminal half of Srv2. Here we describe an equally important role in vivo for the N-terminal half of Srv2 in driving actin turnover. We pinpoint this activity to a conserved patch of surface residues on the N-terminal dimeric helical folded domain of Srv2, and we show that this functional site interacts with cofilin-actin complexes. Furthermore, we show that this site is essential for Srv2 acceleration of cofilin-mediated actin turnover in vitro. A cognate Srv2-binding site is identified on a conserved surface of cofilin, suggesting that this function likely extends to other organisms. In addition, our analyses reveal that higher order oligomerization of Srv2 depends on its N-terminal predicted coiled coil domain and that oligomerization optimizes Srv2 function in vitro and in vivo. Based on these data, we present a revised model for the mechanism by which Srv2 promotes actin turnover, in which coordinated activities of its N- and C-terminal halves catalyze sequential steps in recycling cofilin and actin monomers.

Overexpression of cyclase-associated protein 2 in multistage hepatocarcinogenesis

PURPOSE

Hepatocellular carcinoma (HCC) associated with chronic liver disease is known to show an obvious multistage process of tumor progression. We previously identified heat shock protein 70 as a molecular marker of early HCC during investigation of expression profiling in multistage hepatocarcinogenesis. In this report, we examined cyclase-associated protein 2 (CAP2), which is also listed as an up-regulated gene in early HCC.

EXPERIMENTAL DESIGN

We measured the level of CAP2 mRNA by real-time quantitative PCR. We raised a polyclonal antibody against CAP2 and we confirmed the expression of CAP2 by immunoblotting and immunohistochemistry in HCC cell lines and HCC tissues.

RESULTS

According to real-time quantitative PCR, the level of CAP2 mRNA was up-regulated in early HCC when compared with noncancerous liver tissue, and it was further up-regulated in progressed HCC. We raised a polyclonal antibody against CAP2, which showed a single 53-kDa band of strong intensity in the human HCC cell lines and HCC tissues but only a weak band in the noncancerous liver tissues in Western blot analysis. Immunohistochemical examination of CAP2 revealed its significant overexpression in early HCC when compared with noncancerous and precancerous lesions and in progressed HCC when compared with early HCC.

CONCLUSION

Our findings show that CAP2 is up-regulated in HCC when compared with noncancerous and precancerous lesions. This is the first report that proves that CAP2 is up-regulated in human cancers and that this is possibly related to multistage hepatocarcinogenesis.

The yeast actin cytoskeleton: from cellular function to biochemical mechanism

All cells undergo rapid remodeling of their actin networks to regulate such critical processes as endocytosis, cytokinesis, cell polarity, and cell morphogenesis. These events are driven by the coordinated activities of a set of 20 to 30 highly conserved actin-associated proteins, in addition to many cell-specific actin-associated proteins and numerous upstream signaling molecules. The combined activities of these factors control with exquisite precision the spatial and temporal assembly of actin structures and ensure dynamic turnover of actin structures such that cells can rapidly alter their cytoskeletons in response to internal and external cues. One of the most exciting principles to emerge from the last decade of research on actin is that the assembly of architecturally diverse actin structures is governed by highly conserved machinery and mechanisms. With this realization, it has become apparent that pioneering efforts in budding yeast have contributed substantially to defining the universal mechanisms regulating actin dynamics in eukaryotes. In this review, we first describe the filamentous actin structures found in Saccharomyces cerevisiae (patches, cables, and rings) and their physiological functions, and then we discuss in detail the specific roles of actin-associated proteins and their biochemical mechanisms of action.

Mechanism of oligomerisation of cyclase-associated protein from Dictyostelium discoideum in solution

Cyclase-associated protein (CAP) is a highly conserved modular protein implicated in the regulation of actin filament dynamics and a variety of developmental and morphological processes. The protein exists as a high molecular weight complex in cell extracts and purified protein possesses a high tendency to aggregate, a major obstacle for crystallisation. Using a mutagenesis approach, we show that two structural features underlie the mechanism of oligomerisation in Dictyostelium discoideum CAP. Positively charged clusters on the surface of the N-terminal helix-barrel domain are involved in inter-molecular interactions with the N or C-terminal domains. Abolishing these interactions mainly renders dimers due to a domain swap feature in the extreme C-terminal region of the protein that was previously described. Based on earlier studies with yeast CAP, we also generated constructs with mutations in the extreme N-terminal region of Dictyostelium CAP that did not show significantly altered oligomerisation behaviour. Constructs with mutations in the earlier identified protein-protein interaction interface on the N-terminal domain of CAP could not be expressed as soluble protein. Assessment of the soluble proteins indicates that the mutations did not affect their overall fold. Further studies point to the correlation between stability of full-length CAP with its multimerisation behaviour, where oligomer formation leads to a more stable protein.