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Mass of the postsynaptic density and enumeration of three key molecules
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Contributed by Thomas S. Reese, June 25, 2005

Abstract
The total molecular mass of individual postsynaptic densities (PSDs) isolated from rat forebrain was measured by scanning transmission EM. PSDs had a mean diameter of 360 nm and molecular mass of 1.10 ± 0.36 GDa. Because the mass represents the sum of the molecular masses of all of the molecules comprising a PSD, it becomes possible to derive the number of copies of each protein, once its relative mass contribution is known. Mass contributions of PSD-95, synapse-associated protein (SAP)97, and α-Ca2+/calmodulin-dependent protein kinase II (CaMKII) were determined by quantitative gel electrophoresis of PSD fractions. The number of PSD-95 molecules per average PSD, contributing 2.3% of the mass of the PSD, was calculated to be 300, whereas the number of SAP97 molecules, contributing 0.9% of the mass of the PSD, was 90. The α-CaMKII holoenzymes, which contribute 6% of the mass when brains are homogenized within 2 min of interrupting blood flow, have 80 holoenzymes associated with a typical PSD. When blood flow is interrupted 15 min before homogenization, the average mass of PSDs increases by ≈40%. The additional α-CaMKII associated with PSDs accounts for up to 20% of this mass increase, representing the addition of 100–200 α-CaMKII holoenzymes.
Footnotes
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↵ ¶ To whom correspondence should be addressed. E-mail: treese{at}mbl.edu.
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Author contributions: X.C., R.D.L., and T.S.R. designed research; X.C., L.V., R.D.L., and J.D.P. performed research; T.N., T.M.P., and M.S. contributed new reagents/analytic tools; X.C., L.V., R.D.L., and T.M.P. analyzed data; and X.C., R.D.L., and T.S.R. wrote the paper.
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Abbreviations: AMPAR, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor; CaMKII, Ca2+/calmodulin-dependent protein kinase II; LTP, long-term potentiation; NMDAR, N-methyl-d-aspartate receptor; PSD, postsynaptic density; SAP, synapse-associated protein; STEM, scanning transmission EM; TEM, transmission EM; TMV, tobacco mosaic virus.
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Freely available online through the PNAS open access option.
- Copyright © 2005, The National Academy of Sciences