The metabolic turnover of the major proteins of the postsynaptic density

We have used the method of Austin, Lowry, Brown and Carter, to measure the steady-state metabolic half-life of tubulin (alpha and beta individually) and actin (beta and gamma together) in the total cytosolic (S3), microsomal (P3), synaptic plasma membrane (SPM) and synaptic junction (SJ) subcellular fractions from 6-day-old and adult chicken forebrain. In the SPM and SJ fractions we also measured the steady-state metabolic half-life of the major postsynaptic density protein (mPSDp). In SPM and SJ fractions from 6-day-old chickens tubulin and actin turned over approximately twice as slowly (t1/2 approximately equal to 24 days) as tubulin and actin in the S3 fraction (t1/2 approximately equal to 13 days). This difference was unlikely merely to be due to association with membranes since the t1/2 values for the proteins were the same in P3 and S3. The estimated t1/2 values for mPSDp were similar to that for tubulin and actin in SPM and SJ fractions. Similar results were obtained in adult chickens except that all t1/2 values in all fractions were approximately 30% larger. The calculated t1/2 values did not change between labelling periods of 4 and 6.5 h suggesting that the lag phase of incorporation of newly synthesized PSD proteins is sufficiently rapid to not produce this result artefactually. When the brain from a non-labelled chicken was homogenized in the presence of the S3 fraction from a labelled chicken and sub-fractionated the relative specific activities of the SPM and SJ fractions produced were 1-2% of those from the labelled brain. These results support the notion that tubulin and actin are intrinsic components of the PSD.