Compartmentalization of cold-stable and acetylated microtubules in the subsynaptic domain of chick skeletal muscle fibre.

The junction between motor nerve and skeletal muscle in vertebrates is characterized by a complex subneural structure in which the nicotinic acetylcholine receptor (AChR) accumulates at a surface density of up to 10,000-20,000 molecules per microns 2. Among the mechanisms involved in the maintenance of the accumulation of AChR molecules is the localized expression of AChR subunit genes in the sarcoplasmic nuclei underlying the motor endplate, referred to as 'fundamental' by Ranvier. Immunocytochemical analysis has revealed that in innervated muscle fibres, the Golgi apparatus is also localized to subjunctional domains. We now report an accumulation of cold-stable and acetylated microtubules in the region of the sarcoplasm underlying the endplate. Together, these observations indicate that the subsynaptic sarcoplasm represents a compartment that is specialized both for the transcription, post-translational processing and stabilization of proteins of the postsynaptic membrane, in particular AChR, and for their targeting to the motor endplate.