Common membrane proteins of chromaffin granules, endocrine and synaptic vesicles: Properties, tissue distribution, membrane topography and regulation of synthesis.

The membranes of chromaffin granules, other endocrine vesicles and large dense core vesicles of nerve have many common antigens: cytochrome b-561, GP II and III, carboxypeptidase H, subunits of the proton-pumping ATPase, the amine carrier, PAM and probably also low concentrations of syn, SV2 and p65. One might tend to conclude for these organelles that all these vesicles are equal, however some may still be more equal. Translucent synaptic vesicles of nerve contain higher concentrations of syn than endocrine vesicles. This may also be true for p65, SV2 and synaptobrevin. Apparently endocrine tissues contain, in addition to the hormone-storing organelles, vesicular membranes which resemble synaptic vesicles but not endocrine vesicles in their composition. The function of these special syn containing vesicles in endocrine tissues is obscure. In nerves the biosynthetic relationship between these various vesicles is still far from clear. Two alternative hypotheses are discussed. The function of only some of the membrane antigens is known. In adrenal medulla it has already been established that the biosynthesis of the membrane proteins of chromaffin granules can be turned on in a sophisticated way allowing the synthesis of membranes with quite a changed composition. All this knowledge on molecules and their properties should help us to understand how endocrine tissues and neurones cope with the demanding requirements of complex organisms.