The body wall muscle cells of Caenorhabitis elegans contain an obliquely striated myofibrillar lattice that is associated with the cell membrane through two structures: an Mline analogue in the A-band and a Z-disc analogue, or dense-body, in the I-band. By using a fraction enriched in these structures as an immunogen for hybridoma production, we prepared monoclonal antibodies that identify four components of the I-band as determined by immunofluorescence and Western transfer analysis. A major constituent of the dense-body is a 107,000-D polypeptide that shares determinants with vertebrate a-actinin. A second densebody constituent is a more basic and antigenically distinct 107,000-D polypeptide that is localized to a narrow domain of the dense-body at or subjacent to the plasma membrane. This basic dense-body polypeptide is also found at certain cell boundaries where thin filaments in half-bands terminate at membrane-associated structures termed attachment plaques. A third, unidentified antigen is also found closely apposed to the cell membrane in regions of not only the dense-body and attachment plaque, but also the M-line analogue. Finally, a fourth high molecular weight antigen, composed of two polypeptides of ~400,000-D, is localized to the I-band regions surrounding the dense-body. The attachment of the densebody to the cell surface and the differential localization of the dense-body-associated antigens suggest a model for their organization in which the unidentified antigen is a cell surface component, and the two 107,000-D polypeptides define different cytoplasmic domains of the dense-body.Comparative studies of muscle structure have revealed many distinct muscle types in invertebrates where specialized requirements for contraction appear to be met by modifications of a common design. One such type, found in nematodes and in some other invertebrates, is an obliquely striated musculature whose structure has been described as intermediate between classical striated and smooth muscle types (43, 44). As typified in the body wall muscle of nematodes such as Ascaris (43) and Caenorhabditis elegans (33,60), the distinctive feature of sarcomere organization in this muscle type is that the A-I striations run obliquely to the axes of myofilaments rather than at a 90* angle as in cross-striated muscle. In place of Z-discs, thin filaments attach to dense-body structures and, as in smooth muscle, there are prominent connections between contractile elements and the cell surface at intervals along the entire length of the cell rather than prin-1532 cipaUy at opposite ends of the cell (26, 43).Our interest in C. elegans muscle has arisen as part of investigations of the genetic specification of muscle development and structure in this animal. More than 20 genes that can mutate to affect the normal lattice structure of the C.elegans body wall musculature have already been identified, and the products of several genes have been determined (12,36,(59)(60)(61)68). The most intensively studied gene, unc-54, encodes th...

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Cysteine mutagenesis was used to test the proximity of 16 residues to protein-ligand interaction sites in maltoporin (LamB protein). LamB protein with additional cysteines was incorporated into the outer membrane of Escherichia coli except with a Ser-30----Cys substitution. Phage Lambda and starch binding was assayed before and after incubation of mutants with six thiol-specific reagents. Four categories of mutation were recognized on the basis of phenotype and modification for each of the Lambda- and starch binding sites. The thiol modification experiments helped to clarify whether the phenotype of a mutation was due to a substitution at the binding site or an indirect perturbation of the structure. This study suggests that the cysteine mutagenesis/thiol modification approach may be usefully applied to the operational mapping of surface-accessible binding sites or epitopes.

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