Isolation of mammalian calelectrins: a new class of ubiquitous calcium(2+)-regulated proteins

Abstract

In a new approach to isolating proteins which participate in the Ca2+-dependent regulation of membrane traffic in animal cells, 2 new Ca2+-binding proteins (MW 67,000 and 32,500) were identified in and purified from bovine liver, brain and adrenal medulla. These proteins specifically and reversibly bind to chromaffin granule membranes at low Ca2+ concentrations (half-maximal binding at 5.5 .mu.M Ca2+) and greatly potentiate the Ca2+-induced aggregation of these membranes at higher concentrations (above 10 .mu.M). In the presence of ethylene glycol bis(.beta.-aminoethyl ether)-N,N,N'',N''-tetraacetate, the isolated proteins have Stokes radii of 3.40 nm (MW 67,000) and 2.53 nm (MW 32,500) as estimated by gel filtration and therefore occur as monomers. They are slightly acidic proteins with pI [isoelectric points] of 5.85 and 5.60. In bovine tissues, both proteins and a 3rd protein of MW 35,000 cross-react immunologically with each other and with Torpedo calelectrin (MW 34,000) and are therefore identified as mammalian calelectrins. In all tissues of Torpedo marmorata tested, only a single molecular mass form of calelectrin exists, whereas multiple forms of calelectrin exist in mammalian tissues, indicating gene duplication during evolution. The evolutionary conservation and diversification, the higher tissue concentrations, and the Ca2+-specific interactions of the calelectrins make them candidates for Ca2+-dependent regulators of membrane events in animal cells.