Brain Research, 78 (1974) 83-98 83 © Elsevier ScientificPublishing Company, Amsterdam - Printed in The Netherlands PROTEIN SYNTHESIS IN PHENOTYPICALLY DIFFERENT, SINGLE NEURONS OF APL YSIA Y. PENG LOH* AND R. PRICE PETERSON Departments of Molecular Biology and Anatomy, University of Pennsylvania, Philadelphia, Pa. 19174 (U.S.A.) (Accepted May 8th, 1974) SUMMARY Different molecular weight classes of proteins from single phenotypically differ- ent neuron somas and their subcellular fractions were analyzed in terms of the relative concentration and relative labeling using sodium dodecyl sulfate (SDS) polyacryl- amide microgel electrophoresis. A 12,000 (12K) molecular weight class of protein found in low relative concentration but highly labeled was shown to be present in some endogenously firing neurons, L3, L6, and Lll, but not in others such as L4. A specific 12K class of protein has been found to be common to all neurosecretory neurons, R15, R14 and R3-13, studied. A high relative concentration of 6-9K class of protein in RI 5 and bag cells and a highly labeled 6-9K protein shown to be in R14 and R3-13 have been correlated with neurosecretion. In subcellular fractionation studies a specific 12K class of proteins was found in the membrane fraction of neuron L11. Several proteins, which have molecular weights similar to filarin (77K), tubulin (58-55K) and actin (43K) have been shown,to be common to all neurons studied. Hence this study has demonstrated protein specificity in single phenotypically different neurons. Suggestive correlations of specifically translated small molecular weight proteins with neurosecretion in neurons R3-15 were made. INTRODUCTION The concept that specific macromolecules play a major role in determining neuronal function is fundamental to neurobiology. In order to evaluate this concept further, we have considered the following question: is there protein specificity in * Present address: Behavioral Biology Branch, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Md. 20014, U.S.A.