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Every day and every moment our brains are bombarded by a plethora of information. Neurons in the brain respond to these input information and process it to extract the meaning of the input. Accordingly, the neural nets, which is formed by connection of numerous neurons, flicker on and off. Most of the information arrived at the net pass away. However, certain information is retained in our brain as a memory. Thanks to the plasticity of the synaptic connection, neural net can be modified; the synaptic connection can be either strengthened or weakened and even new nets can be formed. The long-lasting enhancement or depression of the synaptic efficacy is termed long-term potentiation (LTP) or long-term depression (LTD), respectively. Therefore LTP and LTD are regarded as an alphabet of learning and memory. How are the LTP or LTD regulated? Under the manifestation of the physiological phenomena lie hardwares at both sides of the synapse, the presynaptic and postsynaptic machinery. The protein machinery is especially well developed at the postsynaptic side to control the efficacy of signal reception. These megaloproteosome is called the postsynaptic density (PSD). The PSD is a cytoskeletal specialization that is involved in the regulation of synaptic signal tram duction. Unfortunately, characterization of this intriguing structure has been very difficult, mainly due to the hydrophobic nature of the PSD proteins. However, recent development in protein microchemistry and molecular cloning techniques allowed identification of the PSD proteins. As expected, cytoskeletal proteins constitute major-components of the PSD. Recently, other major PSD proteins have been identified by protein sequencing, and their genes were used to fish out associating proteins by two hybrid system expanding our knowledge on the molecular structure of the PSD significantly. In this review, I will focus on proteins that are so far identified.
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