Our understanding of the organization of postsynaptic signaling systems at
excitatory synapses has been aided by the identification of proteins in the
postsynaptic density (PSD) fraction, a subcellular fraction enriched in st
ructures with the morphology of PSDs. In this study, we have completed the
identification of most major proteins in the PSD fraction with the use of a
n analytical method based on mass spectrometry coupled with searching of th
e protein sequence databases. At least one protein in each of 26 prominent
protein bands from the PSD fraction has now been identified. We found 7 pro
teins not previously known to be constituents of the PSD fraction and 24 th
at had previously been associated with the PSD by other methods. The newly
identified proteins include the heavy chain of myosin-Va (dilute myosin), a
motor protein thought to be involved in vesicle trafficking, and the mamma
lian homolog of the yeast septin protein cdc10, which is important for bud
formation in yeast. Both myosin-Va and cdc10 are threefold to fivefold enri
ched in the PSD fraction over brain homogenates. Immunocytochemical localiz
ation of myosin-Va in cultured hippocampal neurons shows that it partially
colocalizes with PSD-95 at synapses and is also diffusely localized in cell
bodies, dendrites, and axons. Cdc10 has a punctate distribution in cell bo
dies and dendrites, with some of the puncta colocalizing with PSD-95. The r
esults support a role for myosin-Va in transport of materials into spines a
nd for septins in the formation or maintenance of spines.