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 (α and β individually) and actin (β and β together) in the total cytosolic (S 3), microsomal (P 3), 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 ( t 1 2 ≈ 24 days ) as tubulin and actin in the S 3 fraction ( t 1 2 ≈ 13 days ). This difference was unlikely merely to be due to association with membranes since the t 1 2 values for the proteins were the same in P 3 and S 3. The estimated t 1 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 t 1 2 values in all fractions were approximately 30% larger. The calculated t 1 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 S 3 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.