ISOASPARTATE IN CHRONDROITIN SULFATE PROTEOGLYCANS OF MAMMALIAN BRAIN

Mammalian brain contains a high mass protein (HMAP) that is unusually rich in atypical L-isoaspartyl (isoAsp) linkages. HMAP has now been pu rified from bovine brain by anion exchange, hydroxylapatite, and size exclusion chromatography, It is self-aggregating, acidic, and soluble in 5% trichloroacetic acid. Treatment with chondroitinase ABC eliminat es the self-aggregation of HMAP and generates several distinct core pr oteins with estimated masses of 350-450 (doublet), 180, and 100 kDa, i ndicating that it is composed mainly of chondroitin sulfate proteoglyc ans (CSPGs), Most of the isoAsp resides in the 350-450-kDa core protei n, which was identified by immunoblotting as phosphacan, a CSPG abunda nt in adult brain. The regional distribution and developmental profile of HMAP in rat brain support this identification. The 180-kDa core pr otein contains a tenascin-R-related molecule, consistent with recent o bservations that phosphacan forms a tight complex with tenascin-R, The average phosphacan molecule in adult brain contains at least seven is oAsp sites. Molecular heterogeneity due to isoAsp may explain some of the complex binding properties phosphacan exhibits with its natural li gands, Formation of isoAsp may be important in the roles that phosphac an and other CSPGs play in development of the nervous system.