Liposomal bupivacaine versus conventional anesthetic or placebo for hemorrhoidectomy: a systematic review and meta-analysis

BACKGROUND

Liposome bupivacaine (LB) is a long-acting anesthetic to enhance postoperative analgesia. Studies evaluating the efficacy of the LB against an active comparator (bupivacaine or placebo) on acute postoperative pain control in hemorrhoidectomy procedures are few and heterogeneous. Therefore, we performed a systematic review and meta-analysis comparing LB's analgesic efficacy and side effects to conventional/placebo anesthetic in hemorrhoidectomy patients.

METHODS

We performed a systematic review and meta-analysis of randomised controlled trials investigating the use of LB after haemorrhoidectomy. We searched the literature published from the time of inception of the datasets to August 19, 2022. The electronic databases included English publications in Ovid MEDLINE In-Process & Other Non-Indexed Citations, Ovid MEDLINE, Ovid EMBASE, and Scopus.

RESULTS

A total of 338 patients who underwent a hemorrhoidectomy procedure enrolled in three randomized clinical trials were included. The overall mean age was 45.84 years (SD ± 11.43), and there was a male predominance (53.55% male). In total 194 patients (52.2%) received LB and 144 (47.8%) received either bupivacaine or placebo. Pain scores at 72 h in the LB (199, 266, and 300 mg) were significantly lower than in the bupivacaine HCl group (p = 0.002). Compared to the bupivacaine/placebo group, the time to first use of opioids in the LB group was significantly longer at LB 199 mg (11 h vs. 9 h), LB 266 mg (19 h vs. 9 h), and LB 300 mg (19 h vs. 8 h) (p < 0.05). Moreover, compared to the bupivacaine/epinephrine group, it was significantly lower in the LB 266 mg group (3.7 vs. 10.2 mg) and at LB 300 mg (13 vs. 33 mg) (p < 0.05). Finally, regarding adverse effects, the conventional anesthetic/placebo group reported more pain in bowel movement than LB groups (OR 2.60, 95% CI 1.31-5.16).

CONCLUSIONS

Comparing LB to conventional anesthetic/placebo anesthetic for hemorrhoidectomy, we found a statistically significant reduction in pain through 72 h, decreased opioid requirements, and delayed time to first opioid use. Moreover, the conventional anesthetic/placebo group reported more pain in bowel movement than LB groups.

Prediction meets time series with gaps: User clusters with specific usage behavior patterns

With mHealth apps, data can be recorded in real life, which makes them useful, for example, as an accompanying tool in treatments. However, such datasets, especially those based on apps with usage on a voluntary basis, are often affected by fluctuating engagement and by high user dropout rates. This makes it difficult to exploit the data using machine learning techniques and raises the question of whether users have stopped using the app. In this extended paper, we present a method to identify phases with varying dropout rates in a dataset and predict for each. We also present an approach to predict what period of inactivity can be expected for a user in the current state. We use change point detection to identify the phases, show how to deal with uneven misaligned time series and predict the user's phase using time series classification. In addition, we examine how the evolution of adherence develops in individual clusters of individuals. We evaluated our method on the data of an mHealth app for tinnitus, and show that our approach is appropriate for the study of adherence in datasets with uneven, unaligned time series of different lengths and with missing values.

The statistical analysis plan for the unification of treatments and interventions for tinnitus patients randomized clinical trial (UNITI-RCT)

BACKGROUND

Tinnitus is a leading cause of disease burden globally. Several therapeutic strategies are recommended in guidelines for the reduction of tinnitus distress; however, little is known about the potentially increased effectiveness of a combination of treatments and personalized treatments for each tinnitus patient.

METHODS

Within the Unification of Treatments and Interventions for Tinnitus Patients project, a multicenter, randomized clinical trial is conducted with the aim to compare the effectiveness of single treatments and combined treatments on tinnitus distress (UNITI-RCT). Five different tinnitus centers across Europe aim to treat chronic tinnitus patients with either cognitive behavioral therapy, sound therapy, structured counseling, or hearing aids alone, or with a combination of two of these treatments, resulting in four treatment arms with single treatment and six treatment arms with combinational treatment. This statistical analysis plan describes the statistical methods to be deployed in the UNITI-RCT.

DISCUSSION

The UNITI-RCT trial will provide important evidence about whether a combination of treatments is superior to a single treatment alone in the management of chronic tinnitus patients. This pre-specified statistical analysis plan details the methodology for the analysis of the UNITI trial results.

TRIAL REGISTRATION

ClinicalTrials.gov NCT04663828 . The trial is ongoing. Date of registration: December 11, 2020. All patients that finished their treatment before 19 December 2022 are included in the main RCT analysis.

Juxtaposing Medical Centers Using Different Questionnaires Through Score Predictors

Background

Chronic tinnitus is a clinically multidimensional phenomenon that entails audiological, psychological and somatosensory components. Previous research has demonstrated age and female gender as potential risk factors, although studies to this regard are heterogeneous. Moreover, whilst recent research has begun to identify clinical “phenotypes,” little is known about differences in patient population profiles at geographically separated and specialized treatment centers. Identifying such differences might prevent potential biases in joint randomized controlled trials (RCTs) and allow for population-specific treatment adaptations.

Method

Two German tinnitus treatment centers were compared regarding pre-treatment data distributions of their patient population bases. To identify overlapping as well as center-specific factors, juxtaposition-, similarity-, and meta-data-based methods were applied.

Results

Between centers, significant differences emerged. One center demonstrated some predictive power of the patients of the other center with regard to questionnaire score after treatment, indicating similarities in treatment response across center populations. Furthermore, adherence to the completion of the questionnaires was found to be an important factor in predicting post-treatment data.

Discussion

Differential age and gender distributions per center should be considered as regards RCT design and individualized treatment planning.

Towards a unification of treatments and interventions for tinnitus patients: The EU research and innovation action UNITI

Tinnitus is the perception of a phantom sound and the patient's reaction to it. Although much progress has been made, tinnitus remains a scientific and clinical enigma of high prevalence and high economic burden, with an estimated prevalence of 10%-20% among the adult population. The EU is funding a new collaborative project entitled "Unification of Treatments and Interventions for Tinnitus Patients" (UNITI, grant no. 848261) under its Horizon 2020 framework. The main goal of the UNITI project is to set the ground for a predictive computational model based on existing and longitudinal data attempting to address the question of which treatment or combination of treatments is optimal for a specific patient group based on certain parameters. Clinical, epidemiological, genetic and audiological data, including signals reflecting ear-brain communication, as well as patients' medical history, will be analyzed making use of existing databases. Predictive factors for different patient groups will be extracted and their prognostic relevance validated through a Randomized Clinical Trial (RCT) in which different patient groups will undergo a combination of tinnitus therapies targeting both auditory and central nervous systems. From a scientific point of view, the UNITI project can be summarized into the following research goals: (1) Analysis of existing data: Results of existing clinical studies will be analyzed to identify subgroups of patients with specific treatment responses and to identify systematic differences between the patient groups at the participating clinical centers. (2) Genetic and blood biomarker analysis: High throughput Whole Exome Sequencing (WES) will be performed in well-characterized chronic tinnitus cases, together with Proximity Extension Assays (PEA) for the identification of blood biomarkers for tinnitus. (3) RCT: A total of 500 patients will be recruited at five clinical centers across Europe comparing single treatments against combinational treatments. The four main treatments are Cognitive Behavioral Therapy (CBT), hearing aids, sound stimulation, and structured counseling. The consortium will also make use of e/m-health applications for the treatment and assessment of tinnitus. (4) Decision Support System: An innovative Decision Support System will be implemented, integrating all available parameters (epidemiological, clinical, audiometry, genetics, socioeconomic and medical history) to suggest specific examinations and the optimal intervention strategy based on the collected data. (5) Financial estimation analysis: A cost-effectiveness analysis for the respective interventions will be calculated to investigate the economic effects of the interventions based on quality-adjusted life years. In this paper, we will present the UNITI project, the scientific questions that it aims to address, the research consortium, and the organizational structure.

Understanding adherence to the recording of ecological momentary assessments in the example of tinnitus monitoring

The recording of Ecological Momentary Assessments (EMA) can assist people with chronic diseases in monitoring their health state. However, many users quickly lose interest in their respective EMA platforms. Therefore, we studied the adherence of users of the mHealth app TrackYourTinnitus (TYT). The app is used to record EMA in people with tinnitus. 1292 users, who interacted with the app between April 2014 and February 2017, were analyzed in this work. We defined “adherence” based on the dimensions of interaction duration and interaction continuity. We propose methods that are able to predict the (dis)continuation of interaction with the app and identify user segments that are characterized by similar patterns of adherence. For the prediction task we used the data of the questionnaires MiniTF and TSCHQ, which are filled in when the users enter TYT for the first time. Additionally, time series of the eight items of the daily EMA questionnaire were used. The distribution of user activity pertaining to the adherence dimension of interaction duration revealed a very skewed distribution, with most users giving up after only 1 day of interaction. However, many users returned after interrupting for some time. Some of the MiniTF items indicated that the worries of users might have lead to an increased likelihood of returning back to the app. The MiniTF score itself was not predictive, though. The answers to the TSCHQ items, in turn, pointed to user strata (more than 65 years of age at registration), which tended towards higher interaction continuity. As the registration questionnaires predicted adherence only to a limited extent, it is promising to study the activities of the users in the very first days of interaction more deeply. It turned out in this context that the effects of interaction stimulants like personalized and non-personalized tips, pointers to information sources, and mechanisms used in online treatments for tinnitus (e.g., in iCBT) should be further investigated.

Directly observed therapy of chronic hepatitis C with ledipasvir/sofosbuvir in people who inject drugs at risk of nonadherence to direct-acting antivirals

An important subgroup of people who inject drugs (PWID) receiving opioid agonist therapy (OAT) cannot be treated in the setting of a hepatology centre and would not regularly ingest their medication when handed to them for self-administration. Our hypothesis was that chronic hepatitis C in these patients could be ideally managed if modern, interferon-free regimens were administered together with OAT under direct observation of a physician or nurse at a low-threshold facility. In this open-label, noninterventional, proof-of-concept study (ClinicalTrials.gov number, NCT02638233), 40 PWID at risk of nonadherence to direct-acting antivirals (DAA) and previously untreated chronic hepatitis C virus genotype 1 infection without cirrhosis were treated with ledipasvir/sofosbuvir for 8 weeks. Patients received antiviral treatment together with OAT under direct observation of a physician or nurse at a low-threshold facility. By following the concept of directly observed therapy, excellent adherence to antiviral therapy was achieved as follows: only 0.16% (95% CI: 0.03-0.47) of scheduled dates for ingestion of the antiviral therapy in combination with OAT were missed by the 40 patients. The rate of sustained virological response 12 weeks after end of therapy was 100% (95% CI: 91.2-100.0). Between week 12 and week 24 of follow-up reinfections were recorded in 2 of 40 patients (5%). Directly observed therapy of chronic hepatitis C is highly effective in PWID at risk of nonadherence to DAA. By this new concept, a group of difficult-to-treat patients can be cured, who could not have been treated in settings of studies published so far.

CAP2, cyclase-associated protein 2, is a dual compartment protein

Cyclase-associated proteins (CAPs) are evolutionarily conserved proteins with roles in regulating the actin cytoskeleton and in signal transduction. Mammals have two CAP genes encoding the related CAP1 and CAP2. We studied the distribution and subcellular localization of CAP1 and CAP2 using specific antibodies. CAP1 shows a broad tissue distribution, whereas CAP2 is significantly expressed only in brain, heart and skeletal muscle, and skin. CAP2 is found in the nucleus in undifferentiated myoblasts and at the M-line of differentiated myotubes. In PAM212, a mouse keratinocyte cell line, CAP2 is enriched in the nucleus, and sparse in the cytosol. By contrast, CAP1 localizes to the cytoplasm in PAM212 cells. In human skin, CAP2 is present in all living layers of the epidermis localizing to the nuclei and the cell periphery. In in vitro studies, a C-terminal fragment of CAP2 interacts with actin, indicating that CAP2 has the capacity to bind to actin.

Formins and VASPs may co-operate in the formation of filopodia

Filopodia are finger-like cell protrusions composed of parallel arrays of actin filaments, which elongate through actin polymerization at their tips. These highly dynamic structures seem to be used by many cell types as sensing organs to explore environmental cues and have been implicated in cell motility as well as in cell-substrate adhesion. Formins are highly conserved multidomain proteins that play important roles in the nucleation of actin and the formation of linear actin filaments, yet their role in filopodia formation has remained poorly defined. The Dictyostelium diaphanous-related formin dDia2 is strongly enriched in filopodia tips. Genetic and biochemical analysis revealed that this protein is important for cell migration and cell adhesion, but most importantly for the formation of filopodia. Recently, we have identified the Dictyostelium VASP (vasodilator-stimulated phosphoprotein) orthologue as a binding partner of dDia2 and provide evidence for a co-operative role of both proteins in filopodia formation.

The genome of the social amoeba Dictyostelium discoideum

The social amoebae are exceptional in their ability to alternate between unicellular and multicellular forms. Here we describe the genome of the best-studied member of this group, Dictyostelium discoideum. The gene-dense chromosomes of this organism encode approximately 12,500 predicted proteins, a high proportion of which have long, repetitive amino acid tracts. There are many genes for polyketide synthases and ABC transporters, suggesting an extensive secondary metabolism for producing and exporting small molecules. The genome is rich in complex repeats, one class of which is clustered and may serve as centromeres. Partial copies of the extrachromosomal ribosomal DNA (rDNA) element are found at the ends of each chromosome, suggesting a novel telomere structure and the use of a common mechanism to maintain both the rDNA and chromosomal termini. A proteome-based phylogeny shows that the amoebozoa diverged from the animal-fungal lineage after the plant-animal split, but Dictyostelium seems to have retained more of the diversity of the ancestral genome than have plants, animals or fungi.

Villin-type headpiece domains show a wide range of F-actin-binding affinities

The villin-type "headpiece" domain is a modular motif found at the extreme C-terminus of larger "core" domains in over 25 cytoskeletal proteins in plants and animals. Although headpiece is classified as an F-actin-binding domain, it has been suggested that some expressed fusion-proteins containing headpiece may lack F-actin-binding in vivo. To determine the intrinsic F-actin affinity of headpiece domains, we quantified the F-actin affinity of seven headpiece domains and three N-terminal truncations, under identical in vitro conditions. The constructs are folded and adopt the native headpiece structure. However, they show a wide range of affinities that can be grouped into high, low, and nonspecific-binding categories. Computer models of the structure and charged surface potential of these headpiece domains suggest features important for high F-actin affinity. We conclude that not all headpiece domains are intrinsically F-actin-binding motifs, and suggest that the surface charge distribution may be an important element for F-actin recognition.

Characterization of CD36/LIMPII homologues in Dictyostelium discoideum

The CD36/LIMPII family is ubiquitously expressed in higher eukaryotes and consists of integral membrane proteins that have in part been characterized as cell adhesion receptors, scavenger receptors, or fatty acid transporters. However, no physiological role has been defined so far for the members of this family that localize specifically to vesicular compartments rather than to the cell surface, namely lysosomal integral membrane protein type II (LIMPII) from mammals and LmpA from the amoeba Dictyostelium discoideum. LmpA, the first described CD36/LIMPII homologue from lower eukaryotes, has initially been identified as a suppressor of the profilin-minus phenotype. We report the discovery and initial characterization of two new CD36/LIMPII-related proteins, both of which share several features with LmpA: (i) their size is considerably larger than that of the CD36/LIMPII proteins from higher eukaryotes; (ii) they show the characteristic "hairpin" topology of this protein family; (iii) they are heavily N-glycosylated; and (iv) they localize to vesicular structures of putative endolysosomal origin. However, they show intriguing differences in their developmental regulation and exhibit different sorting signals of the di-leucine or tyrosine-type in their carboxyl-terminal tail domains. These features make them promising candidates as a paradigm for the study of the function and evolution of the as yet poorly understood CD36/LIMPII proteins.

Dictyostelium discoideum: a genetic model system for the study of professional phagocytes. Profilin, phosphoinositides and the lmp gene family in Dictyostelium

Profilin is a key regulator of actin polymerization, and plays a pivotal role at the interface of the phosphoinositide signal transduction pathway and the cytoskeleton. Recent evidence suggests the involvement of profilin in the regulation of phagocytosis and macropinocytosis, and the transport along the endosomal pathway. Disruption of profilin leads to a complex phenotype that includes abnormal cytokinesis, a block in development and defects in the endosomal pathway. Macropinocytosis, fluid phase efflux and secretion of lysosomal enzymes were reduced, whereas the rate of phagocytosis was increased as compared to wild-type cells. The lmpA gene, a homolog of the CD36/LIMPII family, was identified as a suppressor for most of the profilin-minus defects. This gene encodes an integral membrane protein, it localizes to lysosomes and macropinosomes, and binds to phosphoinositides. Even though phosphatidylinositol lipids constitute only a small fraction of total lipids in the membranes of eukaryotic cells, they play an important role in vesicle transport, signal transduction and cytoskeletal regulation. Disruption of lmpA in wild-type cells resulted in defects in fluid phase efflux and macropinocytosis, but not in phagocytosis. The discovery and initial characterization of two additional members of the CD36/LIMPII family in Dictyostelium, lmpB and lmpC, that exhibit intriguing differences in developmental regulation and their putative sorting signals, suggests that a set of lysosomal integral membrane proteins contribute to the crosstalk between vesicles and cytoskeletal proteins.

Filamins as integrators of cell mechanics and signalling

Filamins are large actin-binding proteins that stabilize delicate three-dimensional actin webs and link them to cellular membranes. They integrate cellular architectural and signalling functions and are essential for fetal development and cell locomotion. Here, we describe the history, structure and function of this group of proteins.

Clinical utility of the modified barium swallow

The purpose of this investigation was to evaluate the immediate and clinically relevant information gained from the modified barium swallow study and to determine the impact of the procedure on patient management. A database containing a nonrandom sample of 608 swallowing studies was reviewed. Results showed that only 10.4% of the studies were classified as normal examinations and aspiration occurred in 32.4%. However, swallowing abnormality without aspiration was recorded in 57.2% of the studies. Five additional outcome variables were assessed: referrals made to other specialties, effectiveness of applied compensatory strategies, treatment recommendations, mode of intake change, and diet grade change. Nearly 83% of the 608 studies showed change in at least one of the variables: needed referral to a specialist was identified on 26.3%; compensatory strategies that improved swallow physiology were identified on 48.4%; swallowing therapy was recommended on 37.2%; changes in mode of intake occurred on 31.4%; and diet texture changes were recommended on 43.8%. The low percentage of normal studies coupled with the high percentage of change in measurable variables indicate high clinical utility for the modified barium swallow study. The misguided tendency to refer to the modified barium study only as a tool for identifying aspiration and the appropriate utilization of the examination for identification of underlying abnormality in swallowing physiology are explained.

Inactivation of lmpA, encoding a LIMPII-related endosomal protein, suppresses the internalization and endosomal trafficking defects in profilin-null mutants

Profilin is a key phosphoinositide and actin-binding protein connecting and coordinating changes in signal transduction pathways with alterations in the actin cytoskeleton. Using biochemical assays and microscopic approaches, we demonstrate that profilin-null cells are defective in macropinocytosis, fluid phase efflux, and secretion of lysosomal enzymes but are unexpectedly more efficient in phagocytosis than wild-type cells. Disruption of the lmpA gene encoding a protein (DdLIMP) belonging to the CD36/LIMPII family suppressed, to different degrees, most of the profilin-minus defects, including the increase in F-actin, but did not rescue the secretion defect. Immunofluorescence microscopy indicated that DdLIMP, which is also capable of binding phosphoinositides, was associated with macropinosomes but was not detected in the plasma membrane. Also, inactivation of the lmpA gene in wild-type strains resulted in defects in macropinocytosis and fluid phase efflux but not in phagocytosis. These results suggest an important role for profilin in regulating the internalization of fluid and particles and the movement of material along the endosomal pathway; they also demonstrate a functional interaction between profilin and DdLIMP that may connect phosphoinositide-based signaling through the actin cytoskeleton with endolysosomal membrane trafficking events.

Mapping the functional surface of domain 2 in the gelsolin superfamily

The crystal structure of the F-actin binding domain 2 of severin, the gelsolin homologue from Dictyostelium discoideum, has been determined by multiple isomorphous replacement and refined to 1.75 A resolution. The structure reveals an alpha-helix-beta-sheet sandwich similar to the domains of gelsolin and villin, and contains two cation-binding sites, as observed in other domain 1 and domain 2 homologues. Comparison of the structures of several gelsolin family domains has identified residues that may mediate F-actin binding in gelsolin domain 2 homologues. To assess the involvement of these residues in F-actin binding, three mutants of human gelsolin domain 2 were assayed for F-actin binding activity and thermodynamic stability. Two of the mutants, RRV168AAA and RLK210AAA, demonstrated a lowered affinity for F-actin, indicating a role for those residues in filament binding. Using both structural and biochemical data, we have constructed a model of the gelsolin domain 1-domain 2-F-actin complex. This model highlights a number of interactions that may serve as positive and negative determinants of filament end- and side-binding.

Cloning and characterization of beta-COP from Dictyostelium discoideum

We have isolated a cDNA coding for beta-COP from Dictyostelium discoideum by polymerase chain reaction using degenerate primers derived from rat beta-COP. The complete cDNA clone has a size of 2.8 kb and codes for a protein with a calculated molecular mass of 102 kDa. Dictyostelium beta-COP exhibits highest homology to mammalian beta-COP, but it is considerably smaller due to a shortened variable region that is thought to form a linker between the highly conserved N- and C-terminal domains. Dictyostelium beta-COP is encoded by a single gene, which is transcribed at moderate levels into two RNAs that are present throughout development. To localize the protein, full-length beta-COP was fused to GFP and expressed in Dictyostelium cells. The fusion protein was detected on vesicles distributed all over the cells and was strongly enriched in the perinuclear region. Based on coimmunofluorescence studies with antibodies directed against the Golgi marker comitin, this compartment was identified as the Golgi apparatus. Beta-COP distribution in Dictyostelium was not brefeldin A sensitive being most likely due to the presence of a brefeldin A resistance gene. However, upon DMSO treatment we observed a reversible disassembly of the Golgi apparatus. In mammalian cells DMSO treatment had a similar effect on beta-COP distribution.

Dictyostelium discoideum: a new host model system for intracellular pathogens of the genus Legionella

The soil amoeba Dictyostelium discoideum is a haploid eukaryote that, upon starvation, aggregates and enters a developmental cycle to produce fruiting bodies. In this study, we infected single-cell stages of D. discoideum with different Legionella species. Intracellular growth of Legionella in this new host system was compared with their growth in the natural host Acanthamoeba castellanii. Transmission electron microscopy of infected D. discoideum cells revealed that legionellae reside within the phagosome. Using confocal microscopy, it was observed that replicating, intracellular, green fluorescent protein (GFP)-tagged legionellae rarely co-localized with fluorescent antibodies directed against the lysosomal protein DdLIMP of D. discoideum. This indicates that the bacteria inhibit the fusion of phagosomes and lysosomes in this particular host system. In addition, Legionella infection of D. discoideum inhibited the differentiation of the host into the multicellular fruiting stage. Co-culture studies with profilin-minus D. discoideum mutants and Legionella resulted in higher rates of infection when compared with infections of wild-type amoebae. Because the amoebae are amenable to genetic manipulation as a result of their haploid genome and because a number of cellular markers are available, we show for the first time that D. discoideum is a valuable model system for studying intracellular pathogenesis of microbial pathogens.

The actin cytoskeleton of Dictyostelium: a story told by mutants

Actin-binding proteins are effectors of cell signalling and coordinators of cellular behaviour. Research on the Dictyostelium actin cytoskeleton has focused both on the elucidation of the function of bona fide actin-binding proteins as well as on proteins involved in signalling to the cytoskeleton. A major part of this work is concerned with the analysis of Dictyostelium mutants. The results derived from these investigations have added to our understanding of the role of the actin cytoskeleton in growth and development. Furthermore, the studies have identified several cellular and developmental stages that are particularly sensitive to an unbalanced cytoskeleton. In addition, use of GFP fusion proteins is revealing the spatial and temporal dynamics of interactions between actin-associated proteins and the cytoskeleton.

Dissection of functional domains by expression of point-mutated profilins in Dictyostelium mutants

Profilin is a ubiquitous cytoskeletal protein whose function is fundamental to the maintenance of normal cell physiology. By site-directed mutagenesis of profilin II from Dictyostelium discoideum the point mutations K114E and W3N were generated by PCR thus changing actin and poly-(L)-proline-binding activity respectively. W3N profilin is no longer able to bind to poly-(L)-proline concomitant with a slight reduction in actin binding. The K114E profilin exhibited a profound decrease in its ability to interact with actin, whereas binding to poly-(L)-proline was essentially unchanged. Binding to phospholipids was indistinguishable from the wild-type profilin. The in vivo properties of the point-mutated profilins were studied by expressing either W3N or K114E in profilin-minus D. discoideum mutants which have defects in the F-actin content, cytokinesis and development (Haugwitz et al., Cell 79, 303-314, 1994). Expression of K114E or W3N displayed a reduction in the F-actin content, normal cell morphology, and the transformants were capable of undergoing complete development. Interestingly, only cells that drastically overexpressed W3N could restore the aberrant phenotype, whereas the mutant protein K114E with its fully functional poly-(L)-proline binding and its strongly reduced actin-binding activities rescued the phenotype at low concentrations. Wild-type and both mutated profilins are enriched in phagocytic cups during uptake of yeast particles. These data suggest a) that a functional poly-(L)-proline-binding activity is more important for suppression of the mutant phenotype than the G-actin binding activity of profilin, and b) that the enrichment of profilin in highly active phagocytic cups might be independent of either poly-(L)-proline or actin-binding activities.

Dictyostelium as model system for studies of the actin cytoskeleton by molecular genetics

The actin cytoskeleton is an essential structure for most movements at the cellular and intracellular level. Whereas for contraction a muscle cell requires a rather static organisation of cytoskeletal proteins, cell motility of amoeboid cells relies on a tremendously dynamic turnover of filamentous networks in a matter of seconds and at distinct regions inside the cell. The best model system for studying cell motility is Dictyostelium discoideum. The cells live as single amoebae but can also start a developmental program that leads to multicellular stages and differentiation into simple types of tissues. Thus, cell motility can be studied on single cells and on cells in a tissue-like aggregate. The ability to combine protein purification and biochemistry with fairly easy molecular genetics is a unique feature for investigation of the cytoskeleton and cell motility. The actin cytoskeleton in Dictyostelium harbours essentially all classes of actin-binding proteins that have been found throughout eukaryotes. By conventional mutagenesis, gene disruption, antisense approaches, or gene replacements many genes that code for cytoskeletal proteins have been disrupted, and altered phenotypes in transformants that lacked one or more of those cytoskeletal proteins allowed solid conclusions about their in vivo function. In addition, tagging the proteins or selected domains with green fluorescent protein allows the monitoring of protein redistribution during cell movement. Gene tagging by restriction enzyme mediated integration of vectors and the ongoing international genome and cDNA sequencing projects offer the chance to understand the dynamics of the cytoskeleton by identification and functional characterisation of all proteins involved.

Assessing the role of the ASP56/CAP homologue of Dictyostelium discoideum and the requirements for subcellular localization

The CAP (cyclase-associated protein) homologue of Dictyostelium discoideum is a phosphatidylinositol 4,5-bisphosphate (PIP(2)) regulated G-actin sequestering protein which is present in the cytosol and shows enrichment at plasma membrane regions. It is composed of two domains separated by a proline rich stretch. The sequestering activity has been localized to the C-terminal domain of the protein, whereas the presence of the N-terminal domain seems to be required for PIP(2)-regulation of the sequestering activity. Here we have constructed GFP-fusions of N- and C-domain and found that the N-terminal domain showed CAP-specific enrichment at the anterior and posterior ends of cells like endogenous CAP irrespective of the presence of the proline rich region. Mutant cells expressing strongly reduced levels of CAP were generated by homologous recombination. They had an altered cell morphology with very heterogeneous cell sizes and exhibited a cytokinesis defect. Growth on bacteria was normal both in suspension and on agar plates as was phagocytosis of yeast and bacteria. In suspension in axenic medium mutant cells grew more slowly and did not reach saturation densities observed for wild-type cells. This was paralleled by a reduction in fluid phase endocytosis. Development was delayed by several hours under all conditions assayed, furthermore, motile behaviour was affected.

Molecular architecture of the rod domain of the Dictyostelium gelation factor (ABP120)

The Dictyostelium discoideum gelation factor is a two-chain actin-cross-linking protein that, in addition to an N-terminal actin-binding domain, has a rod domain constructed from six tandem repeats of a 100-residue motif that has an immunoglobulin fold. To define the architecture of the rod domain of gelation factor, we have expressed in E. coli a series of constructs corresponding to different numbers of gelation factor rod repeats and have characterised them by chemical crosslinking, ultracentrifugation, column chromatography, matrix-assisted laser desorption ionisation (MALDI) mass spectrometry and NMR spectroscopy. Fragments corresponding to repeats 1-6 and 5-6 dimerise, whereas repeats 1-5 and single repeats 3 and 4 are monomeric. Repeat 6 interacts weakly and was present as monomer and dimer when analysed by analytical ultracentrifugation. Proteolytic digestion of rod5-6 resulted in the generation of two polypeptides that roughly corresponded to rod5 and part of rod6. None of these polypeptides formed dimers after chemical crosslinking. Stable dimerisation therefore appears to require repeats 5 and 6. Based on these data a model of gelation factor architecture is presented. We suggest an arrangement of the chains where only the carboxy-terminal repeats interact as was observed for filamin/ABP280, the mammalian homologue of gelation factor.

The crystal structure of the Physarum polycephalum actin-fragmin kinase: an atypical protein kinase with a specialized substrate-binding domain

Coordinated temporal and spatial regulation of the actin cytoskeleton is essential for diverse cellular processes such as cell division, cell motility and the formation and maintenance of specialized structures in differentiated cells. In plasmodia of Physarum polycephalum, the F-actin capping activity of the actin-fragmin complex is regulated by the phosphorylation of actin. This is mediated by a novel type of protein kinase with no sequence homology to eukaryotic-type protein kinases. Here we present the crystal structure of the catalytic domain of the first cloned actin kinase in complex with AMP at 2.9 A resolution. The three-dimensional fold reveals a catalytic module of approximately 160 residues, in common with the eukaryotic protein kinase superfamily, which harbours the nucleotide binding site and the catalytic apparatus in an inter-lobe cleft. Several kinases that share this catalytic module differ in the overall architecture of their substrate recognition domain. The actin-fragmin kinase has acquired a unique flat substrate recognition domain which is supposed to confer stringent substrate specificity.

Identification of a suppressor of the Dictyostelium profilin-minus phenotype as a CD36/LIMP-II homologue

Profilin is an ubiquitous G-actin binding protein in eukaryotic cells. Lack of both profilin isoforms in Dictyostelium discoideum resulted in impaired cytokinesis and an arrest in development. A restriction enzyme-mediated integration approach was applied to profilin-minus cells to identify suppressor mutants for the developmental phenotype. A mutant with wild-type-like development and restored cytokinesis was isolated. The gene affected was found to code for an integral membrane glycoprotein of a predicted size of 88 kD containing two transmembrane domains, one at the NH2 terminus and the other at the COOH terminus. It is homologous to mammalian CD36/LIMP-II and represents the first member of this family in D. discoideum, therefore the name DdLIMP is proposed. Targeted disruption of the lmpA gene in the profilin-minus background also rescued the mutant phenotype. Immunofluorescence revealed a localization in vesicles and ringlike structures on the cell surface. Partially purified DdLIMP bound specifically to PIP2 in sedimentation and gel filtration assays. A direct interaction between DdLIMP and profilin could not be detected, and it is unclear how far upstream in a regulatory cascade DdLIMP might be positioned. However, the PIP2 binding of DdLIMP points towards a function via the phosphatidylinositol pathway, a major regulator of profilin.

Effects of single amino acid substitutions in the actin-binding site on the biological activity of bovine profilin I

For a detailed analysis of the profilin-actin interaction, we designed several point mutations in bovine profilin I by computer modeling. The recombinant proteins were analyzed in vitro for their actin-binding properties. Mutant proteins with a putatively higher affinity for actin were produced by attempting to introduce an additional bond to actin. However, these mutants displayed a lower affinity for actin than wild-type profilin, suggesting that additional putative bonds created this way cannot increase profilin's affinity for actin. In contrast, mutants designed to have a reduced affinity for actin by eliminating profilin-actin bonds displayed the desired properties in viscosity assays, while their binding sites for poly(L)proline were still intact. The profilin mutant F59A, with an affinity for actin reduced by one order of magnitude as compared to wild-type profilin, was analyzed further in cells. When microinjected into fibroblasts, F59A colocalized with the endogenous profilin and actin in ruffling areas, suggesting that profilins are targeted to and tethered at these sites by ligands other than actin. Profilin null cells of Dictyostelium were transfected with bovine wild-type profilin I and F59A. Bovine profilin I, although expressed to only approximately 10% of the endogenous profilin level determined for wild-type Dictyostelium, caused a substantial rescue of the defects observed in profilin null amoebae, as seen by measuring the growth of colony surface areas and the percentage of polynucleated cells. The mutant protein was much less effective. These results emphasize the highly conserved biological function of profilins with low sequence homology, and correlate specifically their actin-binding capacity with cell motility and proliferation.

Importance of a flexible hinge near the motor domain in kinesin-driven motility

Conventional kinesin is a molecular motor consisting of an N-terminal catalytic motor domain, an extended stalk and a small globular C-terminus. Whereas the structure and function of the catalytic motor domain has been investigated, little is known about the function of domains outside the globular head. A short coiled-coil region adjacent to the motor domain, termed the neck, is known to be important for dimerization and may be required for kinesin processivity. We now provide evidence that a helix-disrupting hinge region (hinge 1) that separates the neck from the first extended coiled-coil of the stalk plays an essential role in basic motor activity. A fast fungal kinesin from Syncephalastrum racemosum was used for these studies. Deletion, substitution by a coiled-coil and truncation of the hinge 1 region all reduce motor speed and uncouple ATP turnover from gliding velocity. Insertion of hinge 1 regions from two conventional kinesins, Nkin and DmKHC, fully restores motor activity, whereas insertion of putative flexible linkers of other proteins does not, suggesting that hinge 1 regions of conventional kinesins can functionally replace each other. We suggest that this region is essential for kinesin movement in its promotion of chemo-mechanical coupling of the two heads and therefore the functional motor domain should be redefined to include not only the catalytic head but also the adjacent neck and hinge 1 domains.

Characterization and cloning of a Dictyostelium Ste20-like protein kinase that phosphorylates the actin-binding protein severin

After receiving an external stimulus Dictyostelium amoebae are able to rearrange their actin cytoskeleton within seconds, and phosphorylation is a prime candidate for quick modification of cytoskeletal components. We isolated a kinase from cytosolic extracts that specifically phosphorylated severin, a Ca2+-dependent F-actin fragmenting protein. In gel filtration chromatography severin kinase eluted with a molecular mass of about 300 kDa and contained a 62-kDa component whose autophosphorylation caused a mobility shift in SDS-polyacrylamide gel electrophoresis and stimulated phosphorylation of severin. Severin kinase activity could be specifically precipitated with antibodies raised against the 62-kDa polypeptide. Phosphorylation of severin was strongly reduced in the presence of Ca2+, indicating additional regulation at the substrate level. Peptide sequencing and cloning of the cDNA demonstrated that the 62-kDa protein belongs to the Ste20p- or p21-activated protein kinase family. It is most closely related to the germinal center kinase subfamily with its N-terminal positioned catalytic domain followed by a presumptive regulatory domain at the C terminus. The presence of a Ste20-like severin kinase in Dictyostelium suggests a direct signal transduction from the plasma membrane to the cytoskeleton by phosphorylation of actin-binding proteins.

Crystallization and preliminary X-Ray diffraction characterization of a dimerizing fragment of the rod domain of the Dictyostelium gelation factor (ABP-120)

We have expressed in Escherichia coli a construct corresponding to sequence repeats 5 and 6 of the rod domain of the actin-binding protein Dictyostelium gelation factor (ABP-120). We have obtained orthorhombic P212121 crystals of the protein with a = 43.5 A, b = 103.2 A, c = 124.4 A. These crystals diffract past 2.2 A resolution using synchrotron radiation and are suitable for high-resolution structural analysis. ABP-120 is a key component of the Dictyostelium cytoskeleton, where it functions to crosslink F-actin filaments into networks. This crosslinking function of ABP-120 depends crucially on the formation of dimeric molecules that contain an actin-binding site on each chain, and this dimerization is brought about through interactions between repeating sequence modules in the rod domain. Because the construct we have expressed retains the ability to dimerize, it should enable us to establish the precise manner in which these sequence repeats interact with one another in the intact molecule.

Human keratinocytes maintain reversible anti-apoptotic defenses in vivo and in vitro

Human keratinocytes proliferate and differentiate in an epidermal environment where induction of apoptosis can be triggered by ultraviolet radiation (UVR), activated lymphocytes and cytokines. The purpose of this study was to determine whether keratinocytes were susceptible to apoptosis induced by ionophore, ultra-violet radiation, cytokines or crosslinking of CD95 (Fas/APO-1). In normal human skin exposed to two minimal erythema doses of ultraviolet radiation, suprabasal cells were the first keratinocytes to demonstrate apoptotic nuclei, and by 48 h apoptotic cells were identified throughout the mid to upper epidermis. However, most keratinocytes resisted apoptosis and UVR-induced apoptosis was not observed in basal cells, or in the most differentiated epidermis. Human keratinocytes and keratinocyte cell lines cultured in vitro developed maximal apoptosis 48 h after radiation. Human keratinocytes cultured in full growth factor supplements were resistant to UVR-induced apoptosis compared to keratinocyte cell lines or to a lymphoid cell line (HL60) susceptible to apoptosis. Keratinocyte cell lines were completely resistant to apoptosis induced by interferon-gamma, interferon-alpha, IL-2, IL-6, TNF-alpha, IL-1Ra, and GM-CSF. A subset of the cells in cultures of keratinocytes and transformed keratinocyte cell lines died by apoptosis in response to anti-Fas, IL-1alpha and TNF-alpha plus IFN-gamma and ionophore. Second passage freshly isolated human keratinocytes were much more resistant to apoptosis induced by ionophore, anti-Fas and cytokines than were transformed keratinocyte cell lines. Calcium shift to induce differentiation in second-passage keratinocyte cultures made keratinocytes even more resistant to UVR-induced apoptosis. This parallels the lack of UVR-induced apoptosis observed in the most differentiated keratinocytes in irradiated human skin. Both keratinocytes and keratinocyte cell lines express rather low levels of the anti-apoptotic proteins bcl-2 and bcl-x compared to other apoptosis-resistant cell types. The differences between keratinocytes and keratinocyte cell lines in susceptibility to apoptosis are not explained by difference in expression of bcl-2 or bcl-x. Finally, withdrawal of growth factors from keratinocytes decreased cell survival following UVR and increased the induction of apoptosis. Inhibition of protein synthesis with cyclo-heximide also made keratinocytes more susceptible to UVR-induced apoptosis, indicating that anti-apoptotic defences in cultured keratinocytes are dependent on active protein synthesis. These experiments show that the strong keratinocyte defences against apoptosis are stratified within the epidermis, and can be altered by differentiation and growth factor withdrawal.

The role of the cortical cytoskeleton: F-actin crosslinking proteins protect against osmotic stress, ensure cell size, cell shape and motility, and contribute to phagocytosis and development

We generated Dictyostelium double mutants lacking the two F-actin crosslinking proteins alpha-actinin and gelation factor by inactivating the corresponding genes via homologous recombination. Here we investigated the consequences of these deficiencies both at the single cell level and at the multicellular stage. We found that loss of both proteins severely affected growth of the mutant cells in shaking suspension, and led to a reduction of cell size from 12 microns in wild-type cells to 9 microns in mutant cells. Moreover the cells did not exhibit the typical polarized morphology of aggregating Dictyostelium cells but had a more rounded cell shape, and also exhibited an increased sensitivity towards osmotic shock and a reduced rate of phagocytosis. Development was heavily impaired and never resulted in the formation of fruiting bodies. Expression of developmentally regulated genes and the final developmental stages that were reached varied, however, with the substrata on which the cells were deposited. On phosphate buffered agar plates the cells were able to form tight aggregates and mounds and to express prespore and prestalk cell specific genes. Under these conditions the cells could perform chemotactic signalling and cell behavior was normal at the onset of multicellular development as revealed by time-lapse video microscopy. Double mutant cells were motile but speed was reduced by approximately 30% as compared to wild type. These changes were reversed by expressing the gelation factor in the mutant cells. We conclude that the actin assemblies that are formed and/or stabilized by both F-actin crosslinking proteins have a protective function during osmotic stress and are essential for proper cell shape and motility.

A novel type of protein kinase phosphorylates actin in the actin-fragmin complex

Actin-fragmin kinase (AFK) from Physarum polycephalum specifically phosphorylates actin in the EGTA-resistant 1:1 actin-fragmin complex. The cDNA deduced amino acid sequence reveals two major domains of approximately 35 kDa each that are separated by a hinge-like proline/serine-rich segment of 50 residues. Whereas the N-terminal domain does not show any significant similarity to protein sequences from databases, there are six complete kelch repeats in the protein that comprise almost the entire C-terminal half of the molecule. To prove the intrinsic phosphorylation activity of AFK, full-length or partial cDNA fragments were expressed both in a reticulocyte lysate and in Escherichia coli. In both expression systems, we obtained specific actin phosphorylation and located the catalytic domain in the N-terminal half. Interestingly, this region did not contain any of the known protein kinase consensus sequences. The only known sequence motif present that could have been involved in nucleotide binding was a nearly perfect phosphate binding loop (P-loop). However, introduction of two different point mutations into this putative P-loop sequence did not alter the catalytic activity of the kinase, which indicates an as yet unknown mechanism for phosphate transfer. Our data suggest that AFK belongs to a new class of protein kinases and that this actin phosphorylation might be the first example of a widely distributed novel type of regulation of the actin cytoskeleton in non-muscle cells.

Hisactophilin-mediated binding of actin to lipid lamellae: a neutron reflectivity study of protein membrane coupling

The neutron reflectivity technique is applied to determine the adsorptive interaction of the 13.5-kDa actin-binding protein hisactophilin from Dictyostelium discoideum with lipid monolayers at a lateral pressure of 21 mN/m < or = pi < or = 25 mN/m at the air-water interface. We compare binding of natural hisactophilin exhibiting a myristic acid chain membrane anchor at the N-terminus (DIC-HIS) and a fatty acid-deficient genetic product expressed in Escherichia coli (EC-HIS). It is demonstrated that only the natural hisactophilin DIC-HIS is capable of mediating the strong binding of monomeric actin to the monolayer, where it forms a layer of about 40 A thickness corresponding to the average diameter of actin monomers. Monolayers composed of pure dimyristoyl phosphatidylcholine with fully deuterated hydrocarbon tails and headgroup (DMPC-d67) and 1:1 mixtures of this lipid with chain deuterated dimyristoyl phosphatidylglycerol (DMPG-d54) are studied on subphases consisting either of fully deuterated buffer (D2O) or of a 9:1 H2O/D2O buffer that matches the scattering length density of air (CMA buffer). The reflectivity data are analyzed in terms of layer models, consisting of one to three layers, depending on the contrast of the buffer and the system. We show that both protein species bind tightly to negatively charged 1:1 DMPC-d67/DMPG-d54 monolayers, thereby forming a thin and most probably monomolecular protein layer of 12-15 A thickness. We find that the natural protein (DIC-HIS) partially penetrates into the lipid monolayer, in contrast to chain-deficient species (EC-HIS), which forms only an adsorbed layer. The coverage of the monolayer with DIC-HIS strongly depends on the presence of anionic DMPG in the monolayer. At a bulk protein concentration of 1.5 micrograms/ml, the molar ratio of bound protein to lipid is about 1:45 for the 1:1 lipid mixture but only 1:420 for the pure DMPC.

Structure/function studies on the pH-dependent actin-binding protein hisactophilin in Dictyostelium mutants

Our previous studies have shown that the actin-binding protein hisactophilin from Dictyostelium discoideum is a candidate for organizing the actin cytoskeleton at the plasma membrane in a pH-dependent manner. To further characterize this interaction we isolated hisactophilin overexpression (hisII+) and hisactophilin minus (his-) mutants. D. discoideum contains two hisactophilin isoforms; both genes are independently transcribed and carry a short intron at the same position of the coding region. The deduced amino acid sequence of hisactophilin II showed a characteristic high content of 35 histidine residues out of a total 118 amino acids. After transformation of Dictyostelium AX2 wild-type cells with a genomic fragment designed to inactivate the hisactophilin I gene we obtained hisactophilin II overexpressing mutants (hisII+). Multiple integration of the vector led to strong overexpression of hisactophilin II which even outnumbered the actin concentration by a factor of two. Hisactophilin II protein showed the same biochemical properties as hisactophilin I during purification and in its pH-dependent binding to F-actin; as shown by mass spectrometry the hisactophilin II fraction was almost completely myristoylated despite of this high overexpression. The inactivation of both hisactophilin genes was achieved by gene replacement with a vector construct encompassing parts of gene I and gene II connected by a geneticin cassette. The properties of the hisII+ and his- cells with regard to growth in shaking culture and on Klebsiella plates, development, chemotaxis and morphology were not affected under normal conditions. However, the hisII+ transformants revealed a significant difference to wild-type cells and his- cells when the cytoplasmic pH was lowered by diethylstilbestrol (DES), a proton pump inhibitor. HisII+ cells were more resistant to the acidification; in contrast to AX2 wild-type cells and his- cells they did not form plasma membrane protrusions, showed an increase in F-actin content, and contained large clusters of F-actin. Lowering the internal pH caused an accumulation of hisactophilin below the plasma membrane. The fact that cells deficient in hisactophilin again lose resistance to acidification is in good agreement with the hypothesis that hisactophilin functions as a pH sensor at the plasma membrane by reversibly connecting the membrane with the actin cortical network upon local changes of the proton concentration.

Linking microfilaments to intracellular membranes: the actin-binding and vesicle-associated protein comitin exhibits a mannose-specific lectin activity

Comitin is a 24 kDa actin-binding protein from Dictyostelium discoideum that is located primarily on Golgi and vesicle membranes. We have probed the molecular basis of comitin's interaction with both actin and membranes using a series of truncation mutants obtained by expressing the appropriate cDNA in Escherichia coli. Comitin dimerizes in solution; its principle actin-binding activity is located between residues 90 and 135. The N-terminal 135 'core' residues of comitin contain a 3-fold sequence repeat that is homologous to several monocotyledon lectins and which retains key residues that determine these lectins' three-dimensional structure and mannose binding. These repeats of comitin appear to mediate its interaction with mannose residues in glycoproteins or glycolipids on the cytoplasmic surface of membrane vesicles from D.discoideum, and comitin can be released from membranes with mannose. Our data indicate that comitin binds to vesicle membranes via mannose residues and, by way of its interaction with actin, links these membranes to the cytoskeleton.

Dictyostelium discoideum contains a family of calmodulin-related EF-hand proteins that are developmentally regulated

A full-length genomic DNA fragment that codes for a novel EF-hand protein Dictyostelium discoideum was cloned and sequenced. The protein is composed of 168 amino acids and contains four consensus sequences that are typical for (Ca2+)-binding EF-hand domains. The protein sequence exhibits only minor similarities to other calmodulin-type proteins from Dictyostelium. The genomic DNA harbors two short introns; their positions suggest that the gene is unrelated to the EF-hand proteins from the calmodulin group. Northern blot analysis showed that the mRNA level was significantly increased during development. Polyclonal antibodies raised against the recombinant protein recognized in Western blots a protein of about 20 kDa. Like the mRNA, also the protein was more abundant in developing cells. Overlay experiments with 45Ca2+ indicated that the EF-hands in fact have (Ca2+)-binding activity. The recent description of CBP1, another calmodulin-type Dictyostelium protein that is upregulated during development [Coukell et al. (1995) FEBS Lett. 362, 342-346], suggests that D. discoideum contains a family of EF-hand proteins that have specific functions during distinct steps of development. We therefore designate the protein described in this report as CBP2.

Identification of a cyclase-associated protein (CAP) homologue in Dictyostelium discoideum and characterization of its interaction with actin

In search for novel actin binding proteins in Dictyostelium discoideum we have isolated a cDNA clone coding for a protein of approximately 50 kDa that is highly homologous to the class of adenylyl cyclase-associated proteins (CAP). In Saccharomyces cerevisiae the amino-terminal part of CAP is involved in the regulation of the adenylyl cyclase whereas the loss of the carboxyl-terminal domain results in morphological and nutritional defects. To study the interaction of Dictyostelium CAP with actin, the complete protein and its amino-terminal and carboxyl-terminal domains were expressed in Escherichia coli and used in actin binding assays. CAP sequestered actin in a Ca2+ independent way. This activity was localized to the carboxyl-terminal domain. CAP and its carboxyl-terminal domain led to a fluorescence enhancement of pyrene-labeled G-actin up to 50% indicating a direct interaction, whereas the amino-terminal domain did not enhance. In polymerization as well as in viscometric assays the ability of the carboxyl-terminal domain to sequester actin and to prevent F-actin formation was approximately two times higher than that of intact CAP. The sequestering activity of full length CAP could be inhibited by phosphatidylinositol 4,5-bisphosphate (PIP2), whereas the activity of the carboxyl-terminal domain alone was not influenced, suggesting that the amino-terminal half of the protein is required for the PIP2 modulation of the CAP function. In profilin-minus cells the CAP concentration is increased by approximately 73%, indicating that CAP may compensate some profilin functions in vivo. In migrating D. discoideum cells CAP was enriched at anterior and posterior plasma membrane regions. Only a weak staining of the cytoplasm was observed. In chemotactically stimulated cells the protein was very prominent in leading fronts. The data suggest an involvement of D. discoideum CAP in microfilament reorganization near the plasma membrane in a PIP2-regulated manner.