Analysis of the relative amounts of the acetylcholine receptros (AChR) and of the 43K protein present in the membrane of developing electrocyte shows that massive accumulation of 43K protein is not required for induction of early AChR clustering. Furthermore, we demonstrate the existence o of cytosol- and membrane-associated 43K polypeptide pools in Torpedo electrocyte. Epitope analysis shows that both pools of 43K protein are related to the long mRNA transcript and share similar antigenic determinants distributed throughout the protein sequence. Their partition between the cytosol and membrane fractions abruptly increases in favor of the membrane during the postsynaptic maturation phase of development, supporting a role for 43K protein in the stabilization and maintenance of the postsynaptic domain.

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