CORRELATIVE BIOCHEMICAL AND MORPHOLOGICAL STUDIES OF BRAIN CALSPECTIN: A SPECTRIN-LIKE CALMODULIN-BINDING PROTEIN

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Abstract
A spectrin-like calmodulin-binding protein termed calspectin from a membrane fraction of [bovine] brain was previously purified. Calspectin, composed of .alpha. (240,000-MW) and .beta. (235,000-MW) subunits, is located predominantly in membranes and can be released from them with media used for the extraction of spectrin from erythrocyte ghosts but not with Triton X-100. Biochemical and EM studies show that the calspectin molecule undergoes dimer-tetramer interconversion in solution (existing as a tetramer in 0.1 M KCl and as a dimer in 0.6 M KCl). Although the dimer and tetramer are both bound to actin filaments, only the tetramer can cross-link actin to form a viscous gel. EM images of calspectin molecules obtained by a low angle rotary-shadowing technique show extended, flexible rod-like shapes of 220 nm (tetramer) in length, which is very similar to the reported images of erythrocyte spectrin. The heterodimetric form appears to consist of .alpha. and .beta. monomers lying side by side. The tetrameric form appears to consist of 2 heterodimers joined head-to-head, with the actin binding sites appearing at the tail ends of the dimers. Calspectin and fodrin, which was recently purified and characterized as a spectrin-like calmodulin-binding protein are probably one and the same.

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