Isolation and characterization of sea urchin egg spectrin: Calcium modulation of the spectrin–actin interaction

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Abstract
Sea urchin egg spectrin has been purified from a homogenate of unfertilized Strongylocentrotus purpuratus eggs using standard biochemical procedures. SDS-PAGE analysis of the molecule revealed a closely spaced, high molecular weight doublet at 237/234 kDa (present in an equimolar ratio). Rotary shadowed images of egg spectrin revealed a double-stranded, elongate, flexible rod-shaped contour, measuring 210 nm in length and ∼ 4–8 nm in width. Additionally, this molecule is shown to be immunologically related to avian erythroid spectrin, since it cross-reacts with antibodies prepared against the chicken erythrocyte α-spectrin/240 kDa subunit. The interaction of egg spectrin with actin was examined by sedimentation and falling-ball viscometry assays. The binding and cross linking properties of spectrin to actin demonstrate a unique Ca++-sensitive regulation at micromolar Ca++ concentrations. This observation provides new insight into the way Ca++ may regulate spectrin–actin interactions in vitro and further suggests possible structural and modulatory roles for egg spectrin in the developing sea urchin embryo.