Organization of intracellular calcium regulatory proteins in the neuronal endoplasmic reticulum
The smooth endoplasmic reticulum (SER) has been established as an intracellular calcium storage site in neurons. Although the SER appears to form a continuous membrane system within neurons, immunolocalization studies suggest that calcium regulatory proteins are not evenly distributed within the SER but are selectively concentrated or excluded from certain domains. The subcompartmenalization of the SER has been clearly demonstrated in Purkinje neurons where two proteins involved in the release of calcium from intracellular stores, the IP3 and ryanodine receptor, were differentially localized within dendrites. Both proteins were found associated with the SER in cell bodies and dendrites of chick Purkinje neurons but only labeling for the IP3 receptor was found within dendritic spines. A similar differential localization was described in Purkinje cell dendrites for the SER Ca++ATPase and calsequestrin, a lumenal calcium binding protein. The Ca++ATPase was found throughout dendrites and dendritic spines while calsequestrin was restricted to membranous profiles within the dendritic shaft.