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The spectrin membrane skeleton is a ubiquitous cytoskeletal structure with several cellular roles, including the maintenance of cell integrity, determination of cell shape and as a contributor to cell polarity. We have isolated mutations in the gene encoding βHeavy-spectrin in Drosophila, and have named this essential locus karst. karst mutant individuals have a pleiotropic phenotype characterized by extensive larval lethality and, in adult escapers, rough eyes, bent wings, tracheal defects and infertility. Within karst mutant eyes, a significant number of ommatidia specifically lack photoreceptor R7 alongside more complex morphological defects. Immunolocalization of βHeavy-spectrin in wild-type eye-antennal and wing imaginal discs reveals that βHeavy-spectrin is present in a restricted subdomain of the membrane skeleton that colocalizes with DE-cadherin. We propose a model where normal levels of Sevenless signaling are dependent on tight cell-cell adhesion facilitated by the βHeavy-spectrin membrane skeleton. Immunolocalization of βHeavy-spectrin in the adult and larval midgut indicates that it is a terminal web protein, but we see no gross morphological defects in the adult apical brush border in karst mutant flies. Rhodamine phalloidin staining of karst mutant ovaries similarly reveals no conspicuous defect in the actin cytoskeleton or cellular morphology in egg chambers. This is in contrast to mutations in α-spectrin, the molecular partner of βHeavy-spectrin, which affect cellular structure in both the larval gut and adult ovaries. Our results emphasize the fundamental contribution of the spectrin membrane skeleton to normal development and reveals a critical interplay between the integrity of a cell’s membrane skeleton, the structure of cell-cell contacts and cell signaling.

Many structural, signaling, and adhesion molecules contain tandemly repeated amino acid motifs. The alpha-actinin/spectrin/dystrophin superfamily of F-actin-crosslinking proteins contains an array of triple alpha-helical motifs (spectrin repeats). We present here the complete sequence of the novel beta-spectrin isoform beta(Heavy)-spectrin (beta H). The sequence of beta H supports the origin of alpha- and beta-spectrins from a common ancestor, and we present a novel model for the origin of the spectrins from a homodimeric actin-crosslinking precursor. The pattern of similarity between the spectrin repeat units indicates that they have evolved by a series of nested, nonuniform duplications. Furthermore, the spectrins and dystrophins clearly have common ancestry, yet the repeat unit is of a different length in each family. Together, these observations suggest a dynamic period of increase in repeat number accompanied by homogenization within each array by concerted evolution. However, today, there is greater similarity of homologous repeats between species than there is across repeats within species, suggesting that concerted evolution ceased some time before the arthropod/vertebrate split. We propose a two-phase model for the evolution of the spectrin repeat arrays in which an initial phase of concerted evolution is subsequently retarded as each new protein becomes constrained to a specific length and the repeats diverge at the DNA level. This evolutionary model has general applicability to the origins of the many other proteins that have tandemly repeated motifs.

The banks of Brine Pool NR1, a brine-filled pockmark in the Gulf of Mexico, are host to a population of Bathymodiolus childressi, a hydrocarbon seep mussel with methanotrophic endosymbionts. The mussel community surrounds the pool, covering an area of -540 m2 and can be divided into two distinct zones separated by a transitional area. The inner zone extends inward from the edge of the pool --1 m. The water among the mussels in this zone has high methane concentrations (>200 imol/L) and moderate oxygen concentrations (nondetectable to 161 pxmol/L). Hydrogen sulfide was rarely detected in this zone. The outer zone extends -1 m inward from the outer edge of the mussel bed. This zone also has high methane concentrations in the water among the mussels (>200 jimol/L), but lower average oxygen levels and areas with very high levels of hydrogen sulfide. The two zones are linked with a transitional area, the middle zone, ranging in width from 1 to 4 m, where intermediate environmental conditions are found. The inner zone was relatively homogenous, while spatial heterogeneity was high in the middle and outer zones, both in the characteristics of the mussel population and in their environment. Size-frequency distributions indicate that the inner zone is an area of active recruitment, with very little recruitment occurring in either the middle or outer zones. Physiological condition indices of the mussels were significantly higher in the inner zone than in the outer zone in some years. Comparisons of growth parameters also indicate better growth conditions in the inner zone. Physiological condition and growth did not change between the first two years of the study. However, a significant change in both was detected between 1994 and 1995, suggesting that this deep-sea community is exposed to an unexpected temporal variability in environmental conditions.