Calmodulin fragments

Fig. 12. Tentative model of the signal transduction chain that links the perception of pectic fragments to defense responses in carrot cells. Abbreviations apy, heterotrimeric G protein CaM, calmodulin 4CL, 4-coumarate-CoA ligase CTX, cholera toxin FC, fusicoccine GDP-P-S and GTP-y-S, guanosine 5 -0-(2-thiodiphosphate) and guanosine 5 -0-(3-thiotriphosphate) IP3, 1,4,5-inositol trisphosphate PAL, phenylalanine ammonia-lyase PLC, phospholipase C PR, pathogenesis related PTX, pertussis toxin Rc, receptor SP, staurosporine. Activation and inhibition are symbolized by + and -respectively.

The concentration of Ca2+ required for half-maximal activation of AC1 is approximately 150mmol/l, while AC8 is five times less sensitive. The Clb domain of adenylyl cyclase appears to mediate the activation of the enzyme by Ca2+/calmodulin (see Fig. 21-3). Activation of AC1 by Ca2+/calmodulin can be blocked by a peptide fragment of the Clb portion of this enzyme. This peptide is also capable of binding Ca27calmodulin itself. 

ID-myo-inositol 1,3,4,5-tetrakisphosphate <1> (<1> recombinant, catalytically active fragment of isoform C, allosteric product activation in the absence of Ca /calmodulin, which per se activates enzyme and abolishes the allosteric effect [31]) [31]... 

Finally, the inhibitory effects of phenothiazines on the formation of Of by PMNs have been interpreted as evidence for the participation of calmodulin in the transmission of the signal from the surface of the cell to the oxidase. One action of phenothiazines is to inhibit the effects of the complex of calmodulin and Ca . The activity of the effector molecule, such as phosphodiesterase, which would normally be stimulated by the binary complex of Ca with calmodulin, is not stimulated in the presence of phenothiazines. Jones et al. have shown that in both intact PMNs and in membranous fragments of PMNs stimulated with opsonized zymosan, phenothiazines inhibit the formation of Of. The hierarchy of potency of the various pheno-... 

In the light of the available crystallographic studies, the amino acid sequences and NMR studies, certain general points have been noted for parvalbumin, Wasserman protein, calmodulin, troponin C and SIOO. (1) Each calcium site is formed from residues in a hand which includes a /3-strand. The sites contain backbone carbonyl and side-chain carboxylate. (2) Each binding site is linked to two helices. (3) Each -strand backs on to another strand to form a Ca-/3-sheet-Ca unit, which then involves four helices. (4) The helices interact with each other through largely hydrophobic surfaces. (5) The connections between remote ends of helices are relatively mobile strands, and differ from protein to protein. (6) The four-hand proteins, the calmodulins and troponins, are simileir to the sum of their two-handed fragments. 

Endocytosis may not be required for the entry of an invasive adenylate cyclase from Bordello pertussis (Hanski and Ferfel, 1985 Donovan and Storm, 1990). This is a single chain protein (mol. wt. approx. 200 kDa) which resembles the edema factor from anthrax toxin in that it must interact with calmodulin to become active. In contrast to anthrax toxin, it consists of only one polypeptide which is, however, easily cleaved by proteases and thereby activated. An enzymatically active 45 kDa fragment is not active on whole cells, but it could in conjunction with the rest of the molecule enter the cytosol. The facts that this toxin acts much more rapidly than anthrax toxin, and that it is active even at 4 °C and on erythrocytes that have little, if any, endocytosis, suggest that the toxin is able to penetrate directly through the cell surface membrane. 

An offshoot of this protocol is the limited proteolysis strategy, in which proteolysis is restricted to a single event that cleaves the complex into two complementary fragments only this identifies the most susceptible bond in the protein and hence the protease-accessible amino acid residue. To identify other accessible residues, proteolysis is performed in a time-course fashion with a suite of enzymes of diverse specificities. The potential of the limited proteolysis strategy has been demonstrated by mapping the topology of Ca2+-calmodulin-melittin ternary complex.165... 

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