Hyperunstable matrix proteins in the byssus of Mytilus galloprovincialis.
A previously unidentified endogenous substrate for protein L-isoaspartyl methyltransferase in mammalian brain has been characterized and partially purified. This high mass methyl-accepting protein (HMAP) is concentrated in rat brain cytosol and is not detectable in rat liver, heart, lung, kidney, or skeletal muscle. HMAP is acidic and heterogeneous in size, with an average mass, as judged by size-exclusion high performance liquid chromatography, greater than 700 kDa. After partial purification from cow brain by anion-exchange chromatography, ammonium sulfate fractionation, and gel filtration, HMAP could accept 12.1 nmol of methyl groups per mg of protein, suggesting that it contains a level of isoaspartate at least 50 times greater than that of the average protein in brain cytosol. Partially purified HMAP is degraded by trypsin, verifying that it is composed, at least in part, of protein. Additional studies on this unusual macromolecule may shed important new light on mechanisms of isoaspartate formation in cells and the molecular pathology of brain aging.