Protein half-lives of two subunits of an NMDA receptor-like complex, the 71-kDa glutamate-binding and the 80-kDa CPP-binding protein

We determined the half-lives for two subunits of a complex that functions as a glutamate and N-methyl-D-aspartate (NMDA) receptor-ion channel in synaptic membranes. These two proteins are a 71 kDa glutamate-binding protein (GBP) and an 80 kDa CPP-binding protein (CBP). Seven month-old Fischer 344 rats were injected with L-[¹⁴C] leucine. The radioactivity in the two proteins was determined in a crude synaptosomal membrane fraction obtained from the brains of rats sacrificed from 4 hours to 13 days after the injection. The previously reported data on time-dependent appearance and loss of L-[¹⁴C] leucine radioactivity in the serum was used in the present study to estimate the half-lives of GBP and CBP. Theoretical curves best fit the experimental data obtained for the two proteins assuming apparent half-lives of 14 (± 2.4) and 18 (± 1.2) hours for CBP and GBP, respectively.