Dictyostelium calmodulin: affinity isolation and characterization

Calmodulin-binding proteins in the model organism Dictyostelium: a complete & critical review

Calmodulin is an essential protein in the model organism Dictyostelium discoideum. As in other organisms, this small, calcium-regulated protein mediates a diversity of cellular events including chemotaxis, spore germination, and fertilization. Calmodulin works in a calcium-dependent or -independent manner by binding to and regulating the activity of target proteins called calmodulin-binding proteins. Profiling suggests that Dictyostelium has 60 or more calmodulin-binding proteins with specific subcellular localizations. In spite of the central importance of calmodulin, the study of these target proteins is still in its infancy. Here we critically review the history and state of the art of research into all of the identified and presumptive calmodulin-binding proteins of Dictyostelium detailing what is known about each one with suggestions for future research. Two individual calmodulin-binding proteins, the classic enzyme calcineurin A (CNA; protein phosphatase 2B) and the nuclear protein nucleomorphin (NumA), which is a regulator of nuclear number, have been particularly well studied. Research on the role of calmodulin in the function and regulation of the various myosins of Dictyostelium, especially during motility and chemotaxis, suggests that this is an area in which future active study would be particularly valuable. A general, hypothetical model for the role of calmodulin in myosin regulation is proposed.

Isolation and purification of calmodulin from the shrimp, Crangon crangon

Calmodulin was isolated and purified from shrimp abdominal muscle by heat precipitation, ion exchange and hydrophobic interaction chromatography. The purified calmodulin was homogeneous when evaluated by polyacrylamide gel electrophoresis. A still remaining contaminant was eliminated by high performance liquid chromatography on a phenyl column. The biological and physicochemical properties of shrimp calmodulin such as amino acid composition, molecular weight and the ability to activate calmodulin-deficient bovine heart phosphodiesterase were compared to those of other invertebrate calmodulins.

Two calmodulins in Naegleria flagellates: characterization, intracellular segregation, and programmed regulation of mRNA abundance during differentiation

Flagellates of Naegleria gruberi contain two calmodulins that differ in apparent molecular weight and intracellular location. Calmodulin-1, localized in flagella, has an apparent molecular weight of approximately 16,000, approximately the size of other protozoan calmodulins, whereas calmodulin-2, localized in cell bodies, is 15,300. Both proteins, purified, are calmodulins by several criteria, including Ca2+-dependent stimulation of calmodulin-dependent cyclic nucleotide phosphodiesterase and affinity for antibodies to vertebrate calmodulin. The finding of two calmodulins is unusual. Since the only known difference is apparent molecular weight, one calmodulin could be derived from the other, except that both calmodulins are synthesized in a wheat germ, cell-free system directed by RNA from differentiating Naegleria. Translatable mRNAs encoding calmodulins 1 and 2, not detected in amebas, appear and subsequently disappear concurrently during the 100-min differentiation of Naegleria from amebas to flagellates. Furthermore, these mRNAs increase and then decrease in abundance concurrently with those for flagellar tubulins, which suggests the possibility that the expression of the unrelated genes for calmodulin and tubulin may be under coordinate control during differentiation.

Chemotaxis and cell motility in the cellular slime molds

Chemotaxis and cell motility have essential roles to play throughout the developmental cycle of the cellular slime molds. The particular emphasis of this review, however, will be on the amoeboid stages of the life cycle. The nature of the chemoattractants and their detection will be discussed as will the possible mechanisms that may account for the directed locomotion of amoebae. Intracellular chemoattractant-elicited molecular responses thought to play a role in transduction of extracellular signals into a motility response will also be examined. Furthermore, relationships of these transduction pathway components with changes in assembly states of the cytoskeletal proteins contributing to shape change and cell movement will be assessed. Theories of amoeboid movement involving these cytoskeletal proteins will be compared and discussed in terms of their relevance to cellular slime mold motility.

Comparative structural analysis of calmodulins from Trypanosoma brucei, T. congolense, T. vivax, Tetrahymena thermophila and bovine brain

Calmodulin is an intracellular calcium receptor protein utilized extensively by eukaryotic cells to mediate responsiveness to calcium signals. The present study evaluates the effects on protein structure of amino acid substitutions in trypanosome calmodulin. Calmodulin conformation, hydrophobicity and antigenic determinants are compared among Trypanosoma brucei, Trypanosoma congolense, Trypanosoma vivax, Tetrahymena thermophila and bovine brain. Trypanosome calmodulin differs from brain and Tetrahymena calmodulins based upon isoelectric point, retention time on a C-2/C-18 reverse phase column and interaction with polyclonal antibodies against trypanosome calmodulin by radioimmunoassay or Western procedures. These same analyses do not distinguish trypanosome calmodulins from each other. Polyclonal antibodies against Tetrahymena calmodulin are equally specific and do not recognize the trypanosome or brain calmodulins. Calcium-induced exposure of hydrophobic binding sites are quantitated using the fluorescent probe, N-phenyl-1-naphthylamine. All calmodulins, regardless of source, enhance the fluorescence of N-phenyl-1-naphthylamine 3-4 fold in the presence of calcium. These data demonstrate the extent to which functional calmodulins vary in their structures. We conclude that African trypanosomes share a common calmodulin that is structurally distinct from calmodulin of vertebrates or Tetrahymena.