Importance of phenylalanine residues of yeast calmodulin for target binding and activation

Calmodulin Promotes N-BAR Domain-Mediated Membrane Constriction and Endocytosis

Membrane remodeling by BAR (Bin, Amphiphysin, RVS) domain-containing proteins, such as endophilins and amphiphysins, is integral to the process of endocytosis. However, little is known about the regulation of endocytic BAR domain activity. We have identified an interaction between the yeast Rvs167 N-BAR domain and calmodulin. Calmodulin-binding mutants of Rvs167 exhibited defects in endocytic vesicle release. In vitro, calmodulin enhanced membrane tubulation and constriction by wild-type Rvs167 but not calmodulin-binding-defective mutants. A subset of mammalian N-BAR domains bound calmodulin, and co-expression of calmodulin with endophilin A2 potentiated tubulation in vivo. These studies reveal a conserved role for calmodulin in regulating the intrinsic membrane-sculpting activity of endocytic N-BAR domains.

Calmodulin and STIM proteins: Two major calcium sensors in the cytoplasm and endoplasmic reticulum

The calcium (Ca(2+)) ion is a universal signalling messenger which plays vital physiological roles in all eukaryotes. To decode highly regulated intracellular Ca(2+) signals, cells have evolved a number of sensor proteins that are ideally adapted to respond to a specific range of Ca(2+) levels. Among many such proteins, calmodulin (CaM) is a multi-functional cytoplasmic Ca(2+) sensor with a remarkable ability to interact with and regulate a plethora of structurally diverse target proteins. CaM achieves this 'multi-talented' functionality through two EF-hand domains, each with an independent capacity to bind targets, and an adaptable flexible linker. By contrast, stromal interaction molecule-1 and -2 (STIMs) have evolved for a specific role in endoplasmic reticulum (ER) Ca(2+) sensing using EF-hand machinery analogous to CaM; however, whereas CaM structurally adjusts to dissimilar binding partners, STIMs use the EF-hand machinery to self-regulate the stability of the Ca(2+) sensing domain. The molecular mechanisms underlying the Ca(2+)-dependent signal transduction by CaM and STIMs have revealed a remarkable repertoire of actions and underscore the flexibility of nature in molecular evolution and adaption to discrete Ca(2+) levels. Recent genomic sequencing efforts have uncovered a number of disease-associated mutations in both CaM and STIM1. This article aims to highlight the most recent key structural and functional findings in the CaM and STIM fields, and discusses how these two Ca(2+) sensor proteins execute their biological functions.

Distinctive malfunctions of calmodulin mutations associated with heart RyR2-mediated arrhythmic disease

Calmodulin (CaM) is a cytoplasmic calcium sensor that interacts with the cardiac ryanodine receptor (RyR2), a large Ca(2+) channel complex that mediates Ca(2+) efflux from the sarcoplasmic reticulum (SR) to activate cardiac muscle contraction. Direct CaM association with RyR2 is an important physiological regulator of cardiac muscle excitation-contraction coupling and defective CaM-RyR2 protein interaction has been reported in cases of heart failure. Recent genetic studies have identified CaM missense mutations in patients with a history of severe cardiac arrhythmogenic disorders that present divergent clinical features, including catecholaminergic polymorphic ventricular tachycardia (CPVT), long QT syndrome (LQTS) and idiopathic ventricular fibrillation (IVF). Herein, we describe how two CPVT- (N54I & N98S) and three LQTS-associated (D96V, D130G & F142L) CaM mutations result in alteration of their biochemical and biophysical properties. Ca(2+)-binding studies indicate that the CPVT-associated CaM mutations, N54I & N98S, exhibit the same or a 3-fold reduced Ca(2+)-binding affinity, respectively, versus wild-type CaM, whereas the LQTS-associated CaM mutants, D96V, D130G & F142L, display more profoundly reduced Ca(2+)-binding affinity. In contrast, all five CaM mutations confer a disparate RyR2 interaction and modulation of [(3)H]ryanodine binding to RyR2, regardless of CPVT or LQTS association. Our findings suggest that the clinical presentation of CPVT or LQTS associated with these five CaM mutations may involve both altered intrinsic Ca(2+)-binding as well as defective interaction with RyR2.

Linear Response Path Following: A Molecular Dynamics Method To Simulate Global Conformational Changes of Protein upon Ligand Binding

Molecular functions of proteins are often fulfilled by global conformational changes that couple with local events such as the binding of ligand molecules. High molecular complexity of proteins has, however, been an obstacle to obtain an atomistic view of the global conformational transitions, imposing a limitation on the mechanistic understanding of the functional processes. In this study, we developed a new method of molecular dynamics (MD) simulation called the linear response path following (LRPF) to simulate a protein's global conformational changes upon ligand binding. The method introduces a biasing force based on a linear response theory, which determines a local reaction coordinate in the configuration space that represents linear coupling between local events of ligand binding and global conformational changes and thus provides one with fully atomistic models undergoing large conformational changes without knowledge of a target structure. The overall transition process involving nonlinear conformational changes is simulated through iterative cycles consisting of a biased MD simulation with an updated linear response force and a following unbiased MD simulation for relaxation. We applied the method to the simulation of global conformational changes of the yeast calmodulin N-terminal domain and successfully searched out the end conformation. The atomistically detailed trajectories revealed a sequence of molecular events that properly lead to the global conformational changes and identified key steps of local-global coupling that induce the conformational transitions. The LRPF method provides one with a powerful means to model conformational changes of proteins such as motors and transporters where local-global coupling plays a pivotal role in their functional processes.

A mutation in CALM1 encoding calmodulin in familial idiopathic ventricular fibrillation in childhood and adolescence

OBJECTIVES

This study aimed to identify the genetic defect in a family with idiopathic ventricular fibrillation (IVF) manifesting in childhood and adolescence.

BACKGROUND

Although sudden cardiac death in the young is rare, it frequently presents as the first clinical manifestation of an underlying inherited arrhythmia syndrome. Gene discovery for IVF is important as it enables the identification of individuals at risk, because except for arrhythmia, IVF does not manifest with identifiable clinical abnormalities.

METHODS

Exome sequencing was carried out on 2 family members who were both successfully resuscitated from a cardiac arrest.

RESULTS

We characterized a family presenting with a history of ventricular fibrillation (VF) and sudden death without electrocardiographic or echocardiographic abnormalities at rest. Two siblings died suddenly at the ages of 9 and 10 years, and another 2 were resuscitated from out-of-hospital cardiac arrest with documented VF at ages 10 and 16 years, respectively. Exome sequencing identified a missense mutation affecting a highly conserved residue (p.F90L) in the CALM1 gene encoding calmodulin. This mutation was also carried by 1 of the siblings who died suddenly, from whom DNA was available. The mutation was present in the mother and in another sibling, both asymptomatic but displaying a marginally prolonged QT interval during exercise.

CONCLUSIONS

We identified a mutation in CALM1 underlying IVF manifesting in childhood and adolescence. The causality of the mutation is supported by previous studies demonstrating that F90 mediates the direct interaction of CaM with target peptides. Our approach highlights the utility of exome sequencing in uncovering the genetic defect even in families with a small number of affected individuals.

Surface dynamics in allosteric regulation of protein-protein interactions: modulation of calmodulin functions by Ca2+

Knowledge of the structural basis of protein-protein interactions (PPI) is of fundamental importance for understanding the organization and functioning of biological networks and advancing the design of therapeutics which target PPI. Allosteric modulators play an important role in regulating such interactions by binding at site(s) orthogonal to the complex interface and altering the protein's propensity for complex formation. In this work, we apply an approach recently developed by us for analyzing protein surfaces based on steered molecular dynamics simulation (SMD) to the study of the dynamic properties of functionally distinct conformations of a model protein, calmodulin (CaM), whose ability to interact with target proteins is regulated by the presence of the allosteric modulator Ca(2+). Calmodulin is a regulatory protein that acts as an intracellular Ca(2+) sensor to control a wide variety of cellular processes. We demonstrate that SMD analysis is capable of pinpointing CaM surfaces implicated in the recognition of both the allosteric modulator Ca(2+) and target proteins. Our analysis of changes in the dynamic properties of the CaM backbone elicited by Ca(2+) binding yielded new insights into the molecular mechanism of allosteric regulation of CaM-target interactions.

Specific conformation and Ca(2+)-binding mode of yeast calmodulin: insight into evolutionary development

The vertebrate calmodulin is configured with two structurally independent globular lobes in N- and C-terminus, and a flexible central linker. Distinctly, two lobes of calmodulin from Saccharomyces cerevisiae (yCaM) interact and influence the Ca(2+)-binding profile of each other. We explored this further using the mutant proteins with eliminated Ca(2+)-binding ability in one of the lobes and found that the Ca(2+)-bound N-lobe associates with the Ca(2+)-free C-lobe to gain the Ca(2+) affinity of a wild-type level. Next, analysing series of C-terminal residue truncation mutant, we found that the truncation of C-terminal three residues induce the hyper Ca(2+) affinity. These residues are also important for the general structural behaviour of calmodulin, such as Ca(2+)-induced slow mobility shift in polyacrylamide gel electrophoresis and for the ability to activate Cmk1p (yeast calmodulin kinase). These suggest: (i) when Ca(2+) occupies only N-lobe, two lobes interact and form the stable intermediate leading to a proper level of Ca(2+) affinity; (ii) the C-terminal three residues are required to prohibit abnormal stabilization of the intermediate promoting abnormally high Ca(2+) affinity and for recognition of target enzymes. A model for Ca(2+) and target bindings of yCaM is proposed. Evolutional aspect concerning the biological significance of this model was discussed.

Optimizing ¹⁹F NMR protein spectroscopy by fractional biosynthetic labeling

In protein NMR experiments which employ nonnative labeling, incomplete enrichment is often associated with inhomogeneous line broadening due to the presence of multiple labeled species. We investigate the merits of fractional enrichment strategies using a monofluorinated phenylalanine species, where resolution is dramatically improved over that achieved by complete enrichment. In NMR studies of calmodulin, a 148 residue calcium binding protein, ¹⁹F and ¹H-¹⁵N HSQC spectra reveal a significant extent of line broadening and the appearance of minor conformers in the presence of complete (>95%) 3-fluorophenylalanine labeling. The effects of varying levels of enrichment of 3-fluorophenylalanine (i.e. between 3 and >95%) were further studied by ¹⁹F and ¹H-¹⁵N HSQC spectra, ¹⁵N T(1) and T(2) relaxation measurements, ¹⁹F T(2) relaxation, translational diffusion and heat denaturation experiments via circular dichroism. Our results show that while several properties, including translational diffusion and thermal stability show little variation between non-fluorinated and >95% ¹⁹F labeled samples, ¹⁹F and ¹H-¹⁵N HSQC spectra show significant improvements in line widths and resolution at or below 76% enrichment. Moreover, high levels of fluorination (>80%) appear to increase protein disorder as evidenced by backbone ¹⁵N dynamics. In this study, reasonable signal to noise can be achieved between 60-76% ¹⁹F enrichment, without any detectable perturbations from labeling.

Approaches to the assignment of (19)F resonances from 3-fluorophenylalanine labeled calmodulin using solution state NMR

Traditional single site replacement mutations (in this case, phenylalanine to tyrosine) were compared with methods which exclusively employ (15)N and (19)F-edited two- and three-dimensional NMR experiments for purposes of assigning (19)F NMR resonances from calmodulin (CaM), biosynthetically labeled with 3-fluorophenylalanine (3-FPhe). The global substitution of 3-FPhe for native phenylalanine was tolerated in CaM as evidenced by a comparison of (1)H-(15)N HSQC spectra and calcium binding assays in the presence and absence of 3-FPhe. The (19)F NMR spectrum reveals six resolved resonances, one of which integrates to three 3-FPhe species, making for a total of eight fluorophenylalanines. Single phenylalanine to tyrosine mutants of five phenylalanine positions resulted in (19)F NMR spectra with significant chemical shift perturbations of the remaining resonances, and provided only a single definitive assignment. Although (1)H-(19)F heteronucleclear NOEs proved weak, (19)F-edited (1)H-(1)H NOESY connectivities were relatively easy to establish by making use of the (3)J(FH) coupling between the fluorine nucleus and the adjacent fluorophenylalanine delta proton. (19)F-edited NOESY connectivities between the delta protons and alpha and beta nuclei in addition to (15)N-edited (1)H, (1)H NOESY crosspeaks proved sufficient to assign 4 of 8 (19)F resonances. Controlled cleavage of the protein into two fragments using trypsin, and a repetition of the above 2D and 3D techniques resulted in unambiguous assignments of all 8 (19)F NMR resonances. Our studies suggest that (19)F-edited NOESY NMR spectra are generally adequate for complete assignment without the need to resort to mutational analysis.

Arabidopsis potential calcium sensors regulate nitric oxide levels and the transition to flowering

In plants, flowering is a critical developmental transition orchestrated by four regulatory pathways. Distinct alleles encoding mutant forms of the Arabidopsis potential calcium sensor CML24 cause alterations in flowering time. CML24 can act as a switch in the response to day length perception; loss-of-function cml24 mutants are late flowering under long days, whereas apparent gain of CML24 function results in early flowering. CML24 function is required for proper CONSTANS (CO) expression; components upstream of CO in the photoperiod pathway are largely unaffected in the cml24 mutants. In conjunction with CML23, a related calmodulin-like protein, CML24 also inhibits FLOWERING LOCUS C (FLC) expression and therefore impacts the autonomous regulatory pathway of the transition to flowering. Nitric oxide (NO) levels are elevated in cml23/cml24 double mutants and are largely responsible for FLC transcript accumulation. Therefore, CML23 and CML24 are potential calcium sensors that have partially overlapping function that may act to transduce calcium signals to regulate NO accumulation. In turn, NO levels influence the transition to flowering through both the photoperiod and autonomous regulatory pathways.

Structural basis for the interaction of the myosin light chain Mlc1p with the myosin V Myo2p IQ motifs

Calmodulin, regulatory, and essential myosin light chain are evolutionary conserved proteins that, by binding to IQ motifs of target proteins, regulate essential intracellular processes among which are efficiency of secretory vesicles release at synapsis, intracellular signaling, and regulation of cell division. The yeast Saccharomyces cerevisiae calmodulin Cmd1 and the essential myosin light chain Mlc1p share the ability to interact with the class V myosin Myo2p and Myo4 and the class II myosin Myo1p. These myosins are required for vesicle, organelle, and mRNA transport, spindle orientation, and cytokinesis. We have used the budding yeast model system to study how calmodulin and essential myosin light chain selectively regulate class V myosin function. NMR structural analysis of uncomplexed Mlc1p and interaction studies with the first three IQ motifs of Myo2p show that the structural similarities between Mlc1p and the other members of the EF-hand superfamily of calmodulin-like proteins are mainly restricted to the C-lobe of these proteins. The N-lobe of Mlc1p presents a significantly compact and stable structure that is maintained both in the free and complexed states. The Mlc1p N-lobe interacts with the IQ motif in a manner that is regulated both by the IQ motifs sequence as well as by light chain structural features. These characteristic allows a distinctive interaction of Mlc1p with the first IQ motif of Myo2p when compared with calmodulin. This finding gives us a novel view of how calmodulin and essential light chain, through a differential binding to IQ1 of class V myosin motor, regulate this activity during vegetative growth and cytokinesis.

Binding of calmodulin to Nuf1p is required for karyogamy in Saccharomyces cerevisiae

The role of calmodulin (CaM) during mating in Saccharomyces cerevisiae was examined by using a set of Phe-to-Ala substitutions. We identified ten CaM mutants that exhibited significantly reduced mating efficiencies when crossed to a strain of the opposite mating type harboring the same CaM mutation. Most of the mating-defective CaM mutants were bilateral, i.e., they also exhibited mating defects, albeit minor ones, when crossed to the wild type. When strains carrying different bilateral CaM mutations were mated, the mating efficiencies recovered dramatically. We termed this phenomenon "intragenic mating complementation", and classified the mating-defective CaM mutations into two intragenic mating complementation groups. Two mutant alleles belonging to different groups showed minor defects in cell adhesion and cell fusion, but exhibited severe defects in karyogamy. CaM is known to bind to the essential spindle pole body component Nuf1p. This binding appears to be important for karyogamy because the nuf1(C911R) mutation, which impairs CaM-Nuf1p binding, resulted in a severe defect in karyogamy. Indeed, the two mating-defective CaM mutations were found to compromise formation of the CaM/Nuf1p complex, and the mating defects of these two CaM mutants were suppressible by a dominant, CaM-independent, mutation in NUF1. Taken together, these results suggest that loss of CaM binding to Nuf1p causes a defect in karyogamy, thereby inhibiting productive mating.

The solution structure of apocalmodulin from Saccharomyces cerevisiae implies a mechanism for its unique Ca2+ binding property

We have determined the solution structure of calmodulin (CaM) from yeast (Saccharomyces cerevisiae) (yCaM) in the apo state by using NMR spectroscopy. yCaM is 60% identical in its amino acid sequence with other CaMs, and exhibits its unique biological features. yCaM consists of two similar globular domains (N- and C-domain) containing three Ca(2+)-binding motifs, EF-hands, in accordance with the observed 3 mol of Ca(2+) binding. In the solution structure of yCaM, the conformation of the N-domain conforms well to the one of the expressed N-terminal half-domains of yCaM [Ishida, H., et al. (2000) Biochemistry 39, 13660-13668]. The conformation of the C-domain basically consists of a pair of helix-loop-helix motifs, though a segment corresponding to the forth Ca(2+)-binding site of CaM deviates in its primary structure from a typical EF-hand motif and loses the ability to bind Ca(2+). Thus, the resulting conformation of each domain is essentially identical to the corresponding domain of CaM in the apo state. A flexible linker connects the two domains as observed for CaM. Any evidence for the previously reported interdomain interaction in yCaM was not observed in the solution structure of the apo state. Hence, the interdomain interaction possibly occurs in the course of Ca(2+) binding and generates a cooperative Ca(2+) binding among all three sites. Preliminary studies on a mutant protein of yCaM, E104Q, revealed that the Ca(2+)-bound N-domain interacts with the apo C-domain and induces a large conformational change in the C-domain.

Calmodulin controls organization of the actin cytoskeleton via regulation of phosphatidylinositol (4,5)-bisphosphate synthesis in Saccharomyces cerevisiae

Phosphoinositides regulate a wide range of cellular processes, including proliferation, survival, cytoskeleton remodelling and membrane trafficking, yet the mechanisms controlling the kinases, phosphatases and lipases that modulate phosphoinositide levels are poorly understood. In the present study, we describe a mechanism controlling MSS4, the sole phosphatidylinositol (4)-phosphate 5-kinase in Saccharomyces cerevisiae. Mutations in MSS4 and CMD1, encoding the small Ca(2+)-binding protein calmodulin, confer similar phenotypes, including loss of viability and defects in endocytosis and in organization of the actin cytoskeleton. Overexpression of MSS4 suppresses the growth and actin defects of cmd1-226, a temperature-sensitive calmodulin mutant which is defective in the organization of the actin cytoskeleton. Finally, the cmd1-226 mutant exhibits reduced levels of phosphatidylinositol (4,5)-bisphosphate. These findings suggest that calmodulin positively controls MSS4 activity and thereby the actin cytoskeleton.

Calcium binding is required for calmodulin function in Aspergillus nidulans

To explore the structural basis for the essential role of calmodulin (CaM) in Aspergillus nidulans, we have compared the biochemical and in vivo properties of A. nidulans CaM (AnCaM) with those of heterologous CaMs. Neither Saccharomyces cerevisiae CaM (ScCaM) nor a Ca2+ binding mutant of A. nidulans CaM (1234) interacts appreciably with A. nidulans CaM binding proteins by an overlay assay or activates two essential CaMKs, CMKA and CMKB. In contrast, although vertebrate CaM (VCaM) binds a spectrum of proteins similar to that for AnCaM, it is unable to fully activate CMKA and CMKB, displaying a higher K(CaM) and reduced Vmax for both enzymes. In correlation with the biochemical analysis, neither ScCaM nor 1234 can support A. nidulans growth in the absence of the endogenous protein, whereas VCaM only partially complements the absence of wild-type CaM. Analysis of VCaM and AnCaM chimeras demonstrates that amino acid variations in both N- and C-terminal domains contribute to the inability of VCaM to activate CMKB, but differences in the N terminus are largely responsible for the reduced activity towards CMKA. In vivo, the chimeric molecules support growth equivalently, but only to levels intermediate between those of VCaM and AnCaM, suggesting that the reduced ability to activate the CaMKs is not solely responsible for the inability of VCaM to complement the absence of the wild-type protein. Thus, not only is Ca2+ binding required for CaM function in A. nidulans, but the essential in vivo functions of A. nidulans CaM are uniquely sensitive to the subtle amino acid variations present in vertebrate CaM.

The eukaryotic response regulator Skn7p regulates calcineurin signaling through stabilization of Crz1p

To survive ionic, pH and pheromone stress, the yeast Saccharomyces cerevisiae activates signaling through the Ca2+-activated phosphatase calcineurin to the transcription factor Crz1p/Tcn1p. We show that the overexpression of SKN7, a response-regulator transcription factor, activates transcription from a calcineurin/Crz1p-dependent response element (CDRE). Ca2+-induced, calcineurin/Crz1p-dependent activation of several genes is reduced in skn7 mutants. Skn7p modulates CDRE-dependent transcription by affecting Crz1p protein levels. Specifically, the rate of Crz1p turnover is increased in skn7 mutants. Calcineurin, but not its phosphatase activity, is required for Skn7p-mediated Crz1p stabilization. Skn7p binds to both calcineurin and Crz1p in vitro, and we suggest that this interaction is required for Skn7p regulation of Crz1p. The DNA-binding and internal coiled-coil domains, but not the response- regulator phosphorylation of Skn7p, are necessary for Crz1p-dependent transcriptional activation and Crz1p stabilization by Skn7 in vivo. The DNA-binding domain of Skn7p is also required for binding to Crz1p and calcineurin in vitro. Thus, we propose that Skn7p protects Crz1p from degradation by binding to it and calcineurin through its DNA-binding domain.

Fine structure analysis of the yeast centrin, Cdc31p, identifies residues specific for cell morphology and spindle pole body duplication

Centrin/Cdc31p is a Ca2+-binding protein related to calmodulin found in the MTOC of diverse organisms. In yeast, Cdc31p localizes to the SPB where it interacts with Kar1p and is required for SPB duplication. Recent findings suggest that centrin also functions elsewhere in the cell. To dissect the functions of Cdc31p, we generated cdc31 mutations chosen only for temperature sensitivity, but otherwise unbiased as to phenotype. Three phenotypes of the cdc31 mutants, temperature sensitivity, G2/M arrest, and cell lysis, were not well correlated, indicating that the mutations may differentially affect Cdc31p's interactions with other proteins. Alleles near the C-terminal region exhibited high G2/M arrest and genetic interactions with kar1-Delta17, suggesting that this region modulates an SPB-related function. Alleles causing high lysis and reduced Kic1p kinase activity mapped to the middle of the gene, suggesting disruption of a KIC1-like function and defects in activating Kic1p. A third region conferred temperature sensitivity without affecting cell lysis or G2/M arrest, suggesting that it defines a third function. Mutations in the C-terminal region were also defective for interaction with Kic1p. Mapping the alleles onto a predicted structure of Cdc31p, we have identified surfaces likely to be important for interacting with both Kar1p and Kic1p.

Solution structures of the N-terminal domain of yeast calmodulin: Ca2+-dependent conformational change and its functional implication

We have determined solution structures of the N-terminal half domain (N-domain) of yeast calmodulin (YCM0-N, residues 1-77) in the apo and Ca(2+)-saturated forms by NMR spectroscopy. The Ca(2+)-binding sites of YCM0-N consist of a pair of helix-loop-helix motifs (EF-hands), in which the loops are linked by a short beta-sheet. The binding of two Ca(2+) causes large rearrangement of the four alpha-helices and exposes the hydrophobic surface as observed for vertebrate calmodulin (CaM). Within the observed overall conformational similarity in the peptide backbone, several significant conformational differences were observed between the two proteins, which originated from the 38% disagreement in amino acid sequences. The beta-sheet in apo YCM0-N is strongly twisted compared with that in the N-domain of CaM, while it turns to the normal more stable conformation on Ca(2+) binding. YCM0-N shows higher cooperativity in Ca(2+) binding than the N-domain of CaM, and the observed conformational change of the beta-sheet is a possible cause of the highly cooperative Ca(2+) binding. The hydrophobic surface on Ca(2+)-saturated YCM0-N appears less flexible due to the replacements of Met51, Met71, and Val55 in the hydrophobic surface of CaM with Leu51, Leu71, and Ile55, which is thought to be one of reasons for the poor activation of target enzymes by yeast CaM.

Prenylation of Rho1p is required for activation of yeast 1, 3-beta-glucan synthase

One of the essential protein substrates of geranylgeranyl transferase type I in the budding yeast Saccharomyces cerevisiae is a rho-type GTPase, Rho1p, which is a regulatory subunit of 1, 3-beta-glucan synthase. Previous studies have indicated that modification of Rho1p is significantly reduced in a mutant of the beta subunit of geranylgeranyl transferase type I called cal1-1. Here we present genetic and biochemical evidence showing that modification of Rho1p is required for activity of 1,3-beta-glucan synthase. The 1,3-beta-glucan synthase activity of the cal1-1 membrane was significantly reduced compared with that of the wild-type membrane. The impaired activity was partly due to the reduced amount of Fks1p, a putative catalytic subunit of 1, 3-beta-glucan synthase, but also partly due to reduced affinity between unmodified Rho1p and Fks1p. Glutathione S-transferase (GST)-Rho1 proteins with or without the C-terminal motif required for the modification were purified and used to analyze the interaction. The modified form of GST-Rho1p was specifically able to restore the 1,3-beta-glucan synthase of the rho1-3 membrane. Gel overlay analysis indicated that an unmodified form of GST-Rho1p fails to interact with Fks1p. These results indicated that the geranylgeranylation of Rho1p is a prerequisite to the assembly and activation of 1,3-beta-glucan synthase in vitro. Increased cytoplasmic levels of divalent cations such as Ca(2+) restored both Rho1p modification and the 1,3-beta-glucan synthase activity of cal1-1, suggesting that cytoplasmic levels of the divalent cations affect geranylgeranyl transferase type I activity in vivo.

Survey of the 1998 optical biosensor literature

The utilization of optical biosensors to study molecular interactions continues to expand. In 1998, 384 articles relating to the use of commercial biosensors were published in 130 different journals. While significant strides in new applications and methodology were made, a majority of the biosensor literature is of rather poor quality. Basic information about experimental conditions is often not presented and many publications fail to display the experimental data, bringing into question the credibility of the results. This review provides suggestions on how to collect, analyze and report biosensor data.

Analysis of molecular recognition using optical biosensors

Optical biosensors have made the analysis of molecular interactions rapid and convenient. The field is still young, with commercial instrumentation having been available for less than ten years. For this reason instrument development is still an important factor and the past year has seen continuing advances in instrumentation and particularly in novel sensor surfaces.