Monoclonal antibodies to intermediate filaments in chick muscle cell cultures

Cell and molecular biology of myoblast fusion

In organisms from Drosophila to mammals, the musculature is comprised of an elaborate array of distinct fibers that are generated by the fusion of committed myoblasts. These muscle fibers differ from each other in features that include location, pattern of innervation, site of attachment, and size. The sizes of the newly formed muscles of an embryo are controlled in large part by the number of cells that form the syncitial fiber. Over the past few decades, an extensive body of literature has described the process of myoblast fusion in vertebrates, relying primarily on the strengths of tissue culture model systems. More recently, genetic studies in Drosophila embryos have provided new insights into the process. Together, these studies define the steps necessary for myoblast differentiation, the acquisition of fusion competence, the recognition and adhesion between myoblasts, and the fusion of two lipid bilayers into one. In this review, we have attempted to combine insights from both Drosophila and vertebrate studies to trace the processes and molecules involved in myoblast fusion. Implicit in this approach is the assumption that fundamental aspects of myoblast fusion will be similar, independent of the organism in which it is occurring.

Monoclonal antibody studies of creatine kinase. The ART epitope: evidence for an intermediate in protein folding

Chemical cleavage at cysteine residues with nitrothiocyanobenzoic acid shows that the last 98 amino acids of the 380-amino-acid sequence of chick muscle creatine kinase are sufficient for binding of the monoclonal antibody CK-ART. Removal of the last 30 amino acids by cleavage at methionine residues with CNBr results in loss of CK-ART binding. CK-ART binding is also lost when these C-terminal methionine residues are oxidized to sulphoxide, but binding is regained on reduction. Proteinase K 'nicks' native CK at a single site near the C-terminus and two fragments of 327 amino acides and 53 amino acids can be separated by subsequent SDS or urea treatment. CK-ART still binds normally to 'nicked' CK, which is enzymically inactive. After treatment with either urea (in a competition enzyme-linked immunosorbent assay) or SDS (on Western blots), however, CK-ART binds to neither of the two fragments, although these treatments do not affect binding to intact CK. This suggests that parts of both CK fragments contribute to the CK-ART epitope. CK-ART is both species- and isoenzyme-specific, binding only to chick M-CK. The only C-terminal regions containing chick-specific sequences are residues 300-312 and residues 368-371, the latter group being close to the essential methionine residues. We suggest that one, or possibly both, of these regions is involved in forming the conformational epitope on the surface of the CK molecule which CK-ART recognizes. Native CK is resistant to trypsin digestion. The C-terminal half of urea-treated and partly-refolded CK is also resistant to trypsin digestion, whereas the N-terminal half is readily digested. The results suggest a C-terminal region which can refold more rapidly than the rest of the CK molecule and provide evidence for an intermediate in CK refolding.

Monoclonal antibody studies of creatine kinase. Antibody-binding sites in the N-terminal region of creatine kinase and effects of antibody on enzyme refolding

(1) The binding sites of two monoclonal antibodies, CK-2A7 and CK-5H5, have been located to a 60-amino-acid sequence in the N-terminal region of creatine kinase (CK) by the use of chemical cleavage with formic acid (which cleaves proteins at Asp-Pro bonds) and cyanogen bromide (which cleaves at Met residues). (2) A simple method for preparing chemically-cleaved fragments of proteins for electrophoresis and Western blotting is described. (3) Binding studies with CK preparations from different animal species show that single amino acid changes at residues 39 or 82 prevent binding of CK-2A7 and CK-5H5 respectively. We suggest that Lys-39 and Glu-82 form parts of the binding sites on CK for the two monoclonal antibodies. The two sites lie in variable regions at each end of a highly-conserved sequence (residues 46 to 79) and are inaccessible to antibody in the native enzyme. (4) One of the antibodies, CK-2A7, inhibits the refolding of CK to native enzyme after denaturation by urea.

The dynamic state of heat shock proteins in chicken embryo fibroblasts

Subcellular fractionation and immunofluorescence microscopy have been used to study the intracellular distributions of the major heat shock proteins, hsp 89, hsp 70, and hsp 24, in chicken embryo fibroblasts stressed by heat shock, allowed to recover and then restressed. Hsp 89 was localized primarily to the cytoplasm except during the restress when a portion of this protein concentrated in the nuclear region. Under all conditions, hsp 89 was readily extracted from cells by detergent. During stress and restress, significant amounts of hsp 70 moved to the nucleus and became resistant to detergent extraction. Some of this hsp 70 was released from the insoluble form in an ATP-dependent reaction. Hsp 24 was confined to the cytoplasm and, during restress, aggregated to detergent-insoluble perinuclear phase-dense granules. These granules dissociated during recovery and hsp 24 could be solubilized by detergent. The nuclear hsps reappeared in the cytoplasm in cells allowed to recover at normal temperatures. Sodium arsenite also induces hsps and their distributions were similar to that observed after a heat shock, except for hsp 89, which remained cytoplasmic. We also examined by immunofluorescence the cytoskeletal systems of chicken embryo fibroblasts subjected to heat shock and found no gross morphological changes in cytoplasmic microfilaments or microtubules. However, the intermediate filament network was very sensitive and collapsed around the nucleus very shortly after a heat shock. The normal intermediate filament morphology reformed when cells were allowed to recover from the stress. Inclusion of actinomycin D during the heat shock--a condition that prevents synthesis of the hsps--did not affect the intermediate filament collapse, but recovery of the normal morphology did not occur. We suggest that an hsp(s) may aid in the formation of the intermediate filament network after stress.