The calmodulin regulatory system

Domains responsible for constitutive and Ca(2+)-dependent interactions between calmodulin and small conductance Ca(2+)-activated potassium channels

Small conductance Ca(2+)-activated potassium channels (SK channels) are coassembled complexes of pore-forming SK alpha subunits and calmodulin. We proposed a model for channel activation in which Ca2+ binding to calmodulin induces conformational rearrangements in calmodulin and the alpha subunits that result in channel gating. We now report fluorescence measurements that indicate conformational changes in the alpha subunit after calmodulin binding and Ca2+ binding to the alpha subunit-calmodulin complex. Two-hybrid experiments showed that the Ca(2+)-independent interaction of calmodulin with the alpha subunits requires only the C-terminal domain of calmodulin and is mediated by two noncontiguous subregions; the ability of the E-F hands to bind Ca2+ is not required. Although SK alpha subunits lack a consensus calmodulin-binding motif, mutagenesis experiments identified two positively charged residues required for Ca(2+)-independent interactions with calmodulin. Electrophysiological recordings of SK2 channels in membrane patches from oocytes coexpressing mutant calmodulins revealed that channel gating is mediated by Ca2+ binding to the first and second E-F hand motifs in the N-terminal domain of calmodulin. Taken together, the results support a calmodulin- and Ca(2+)-calmodulin-dependent conformational change in the channel alpha subunits, in which different domains of calmodulin are responsible for Ca(2+)-dependent and Ca(2+)-independent interactions. In addition, calmodulin is associated with each alpha subunit and must bind at least one Ca2+ ion for channel gating. Based on these results, a state model for Ca2+ gating was developed that simulates alterations in SK channel Ca2+ sensitivity and cooperativity associated with mutations in CaM.

Immunocytochemical localization of a calmodulinlike protein in Bacillus subtilis cells

To determine possible functions of the calmodulinlike protein of Bacillus subtilis, the time course of its expression during sporulation and its cellular localization were studied. The protein was expressed in a constitutive manner from the end of logarithmic growth through 8 h of sporulation as determined by antibody cross-reactivity immunoblots and enzyme-linked immunosorbent assays (ELISAs). In partially purified extracts, the immunopositive protein comigrated upon electrophoresis with a protein which selectively bound [(45)Ca]CaCl(2), ruthenium red, and Stains-all. Previous studies showed increased extractability of the calmodulinlike protein from B. subtilis cells when urea and 2-mercaptoethanol were used in breakage buffers, implying that the protein might be partially associated with the membrane fraction. This was confirmed by demonstrating that isolated membrane vesicles of B. subtilis also gave positive immunological tests with Western blotting and ELISAs. To more precisely locate the protein in cells, thin sections of late-log-phase cells, sporulating cells, and free spores were reacted first with bovine brain anticalmodulin specific antibodies and then with gold-conjugated secondary antibodies; the thin sections were examined by transmission electron microscopy. The calmodulinlike protein was found almost exclusively associated with the cell envelope of these fixed, sectioned cells. A possible function of the calmodulinlike protein in sensing calcium ions or regulating calcium ion transport is suggested.

Dynamic redistribution of calmodulin in HeLa cells during cell division as revealed by a GFP-calmodulin fusion protein technique

It has been suggested by many studies that Ca2+ signaling plays an important role in regulating key steps in cell division. In order to study the down stream components of calcium signaling, we have fused the gene of calmodulin (CaM) with that of green fluorescent protein (GFP) and expressed it in HeLa cells. The GFP-CaM protein was found to have similar biochemical properties as the wild-type CaM, and its distribution was also similar to that of the endogenous CaM. Using this GFP-tagged CaM as a probe, we have conducted a detailed examination of the spatial- and temporal-dependent redistribution of calmodulin in living mammalian cells during cell division. Our major findings are: (1) high density of CaM was found to distribute in two sub-cellular locations during mitosis; one fraction was concentrated in the spindle poles, while the other was concentrated in the sub-membrane region around the cell. (2) The sub-membrane fraction of CaM became aggregated at the equatorial region where the cleavage furrow was about to form. The timing of this localized aggregation of CaM was closely associated with the onset of cytokinesis. (3) Using a TA-CaM probe, we found that the sub-membrane fraction of CaM near the cleavage furrow was selectively activated during cell division. (4) When we injected a CaM-specific inhibitory peptide into early anaphase cells, cytokinesis was either blocked or severely delayed. These findings suggest that, in addition to Ca2+ ion, CaM may represent a second signal that can also play an active role in determining the positioning and timing of the cleavage furrow formation.

The increase of calmodulin in PC12 cells induced by NGF is caused by differential expression of multiple mRNAs for calmodulin

A rat pheochromocytoma cell line (PC12 cells) was used as a model to investigate the role of calmodulin and its multiple mRNAs in NGF-induced neuronal differentiation. The effect of NGF on the degree of differentiation was assayed using a simple differentiation scoring system. Significant increases in the differentiation score were seen by one day, and the scores increased about 10-fold by 8 days of treatment. NGF also increased calmodulin in the PC12 cells; significant increases were seen by 2 days of treatment, and a maximum increase of 3-fold was seen by 4 days. Northern blot analysis using a calmodulin riboprobe revealed that all five calmodulin mRNAs found in rat tissue were present in PC12 cells. The relative abundance of the calmodulin mRNAs was 1.7 greater than 1.4 greater than 2.3 greater than 4.1 greater than 0.9 kb. NGF treatment caused a differential increase in these mRNAs. The 1.4 kb transcript (from Gene II) was increased earlier (at 1 day) and to a greater extent (3-fold) than any of the other mRNAs. Studies of the half-lives (t1/2) of these mRNAs suggested that the t1/2 varied with the mRNA; the smaller the mRNA, the shorter the t1/2. However, there were no significant effects of NGF on the t1/2 of any of the mRNAs. These studies indicate that NGF elevates calmodulin in PC12 cells by causing a differential increase in the multiple mRNAs for calmodulin and that the increase in calmodulin may play some part in NGF-induced neuronal differentiation in PC12 cells.

B-50 (GAP-43): biochemistry and functional neurochemistry of a neuron-specific phosphoprotein

The biochemistry and functional neurochemistry of the synaptosomal plasma membrane phosphoprotein B-50 (GAP-43) are reviewed. The protein is putatively involved in seemingly diverse functions within the nervous system, including neuronal development and regeneration, synaptic plasticity, and formation of memory and other higher cognitive behaviors. There is a considerable amount of information concerning the spatial and temporal localization of B-50 (GAP-43) in adult, fetal, and regenerating nervous tissue but far less is known about the physical chemistry and biochemistry of the protein. Still less information is available about posttranslational modifications of B-50 (GAP-43) that may be the basis of neurochemical mechanisms that could subsequently permit a variety of physiological functions. Hence, consideration is given to several plausible roles for B-50 (GAP-43) in vivo, which are discussed in the context of the cellular localization of the protein, significant posttranslational enzymes, and regulatory proteins, including protein kinases, phosphoinositides, calmodulin, and proteases.

Molecular mechanisms of tubulointerstitial hypertrophy and hyperplasia

Adult kidneys, which are principally composed of tubulointerstitium, do not normally regenerate or expand their working pool of functional cells at a very high rate. Loss of kidney tissue, however, can lead to some compensatory renal enlargement. The catalytic forces initiating such exchanges have not been fully articulated by current experimental endeavors. Increasing evidence, nevertheless, does suggest that factors other than simple changes in renal hemodynamics may be involved in this process. Different cellular elements in the tubulointerstitial microenvironment probably modulate changes in tubular enlargement or size through a complex cytokine network. Autocrine and paracrine stimulation of enlargement by different local growth factors also seem to play a pivotal role. After binding to cellular receptors, these factors activate signal transduction pathways resulting in expression of immediate early genes, which by themselves can synchronize the expression of subsequent genes through the medium of transacting factors. The renal enlargement response can also be modified by endocrine hormones that can activate such genes directly and/or stimulate other adjunctive processes, like receptor expression for the regional binding of growth factors. Furthermore, renal enlargement is under negative feedback of inhibitory factors like TGF beta. It is possible, for example, that special genes exist which are only expressed to arrest enlargement. It has been further suggested that activation of the Na+/H+ antiporter is a common denominator in renal enlargement. Recent findings, however, indicate that the activation of this antiporter is not always necessary, and might rather be a parallel event rather than a key phenomena in tubular enlargement. G0/G1 transition of tubular cells seems to involve similar factors in tubular hypertrophy and hyperplasia. The factors which are responsible for the final determination of the enlargement pattern (hypertrophy vs. proliferation) are unknown. The separation between hypertrophy and hyperplasia, although suggested by striking differences in cellular regulation, may be somewhat artificial, since responses leading to tubular enlargement also exist in circumstances where hyperplasia and hypertrophy are combined events. Recently it has been proposed that growth factors stimulate gluconeogenesis in proximal tubular cells producing hyperplasia, whereas factors inhibiting gluconeogenesis might induce hypertrophy. Whether the common pathway message of this intriguing hypothesis is correct still requires further validation.(ABSTRACT TRUNCATED AT 400 WORDS)

Calmodulin-binding proteins as calpain substrates

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The interaction of delta-hemolysin with calmodulin

Delta-Hemolysin forms a 1:1 complex with Ca2+ -liganded calmodulin. Probably because of the pronounced tendency of delta-hemolysin to self-associate, the apparent binding affinity is much less than that for melittin. Complex formation is reflected by an increase in quantum yield of Trp-15 of delta-hemolysin and by increased shielding from acrylamide quenching. There is, however, no indication of a change in peptide molecular ellipticity. The binding of 2-toluidinyl-naphthalene-6-sulfonate is reduced by complex formation, suggesting the involvement of a hydrophobic region. Complex formation also blocks the proteolysis by trypsin of the bond between residues 77 and 78. The time decays of fluorescence intensity and anisotropy for tryptophan are multiexponential for both free and complexed delta-hemolysin; the average decay time for intensity is substantially increased for the complex. The localized mobility of tryptophan is greatly reduced in the complex. Complex formation appears to involve both the C-terminal lobe and the connecting strand of calmodulin.