Recently, we have characterized a membrane-bound (mb) component of brain protein carboxylmethyltransferase II (PCMT) which effectively carboxylmethylates endogenous mb methyl-accepting proteins (MAPs). (Neurochem. Int., 10 (1987) 155). We have also shown that exposing mb-MAPs to mild alkali leads to a marked increase in their recognition by PCMT. Since one of the likely consequences of the alkaline treatment appears to be the deamidation of selected protein-bound asparagines or aspartates, followed by the formation, in their place, of D-or L-isoaspartates, it is reasonable to assume that mb-MAPs constitute unique targets for the mb-PCMT because they contain such unnatural aspartate residues. Testing the relevance of this notion to the aging of cerebral mb-MAPs we focus in this report on age-related changes involving mb-MAPs. When two-or six-times washed (in 50 mM NaPO4 buffer, pH 6.5) 17,500 g, 30-min membranes or Percoll-gradient purified synaptic membranes were prepared from young (3-4 months) and old (11-12 months) rat brains and were incubated with 20 microM [3H]methyl S-adenosyl-L-methionine at pH 6.0, mb-MAP carboxyl[3H]methylation was significantly more intense in the old than in the young membranes, no additional increase being noted at 28-35 months. Mb-MAP carboxylmethylation increases were confirmed over a wide range of membrane protein concentrations and incubation times and are taken to reflect age-related modifications of the primary structure of susceptible mb-MAPs. To investigate these, we incubated young and old membranes, as well as their Lubrol-Px (1%) extracts (30 min, 0 degree C), with 0.05 M NH4OH for 90 min at 37 degrees C, a treatment which left PCMT activity largely unaffected. Our findings reveal that the effect of the NH4OH treatment on the generation of carboxylmethylatable sites was markedly smaller in "old" than in "young" proteins, suggesting that "new" carboxylmethylatable sites are generated in susceptible mb-MAPs in situ, by a process accompanying, or otherwise marking, the natural aging of neural membrane proteins.