Involvement of a specific protein in the regulation of a circadian rhythm in Aplysia eye

Our previous results indicated that protein synthesis was necessary for serotonin (5-hydroxytryptamine, 5-HT) to regulate the phase of the biological clock in the Aplysia eye. Also, we showed that 5-HT appeared to increase the synthesis of a 34-kDa protein with an isoelectric point of 7.2. Subsequent studies were carried out to quantitate the effect of 5-HT on the 34-kDa protein and to examine whether the 34-kDa protein was involved in the circadian timing system. The regional specificity of the effect of 5-HT on the 34-kDa protein was investigated. The proximal portion of the eye appeared to synthesize much more of the 34-kDa protein than the distal portion. Also, 5-HT had a much larger effect on the synthesis of the 34-kDa protein in the proximal portion than in the distal portion. The proximal location of synthesis and the 5-HT effect on the synthesis of the 34-kDa protein correlate with the proximal location of cells and processes that are necessary for the expression of the circadian rhythm. The relationship between the effect of 5-HT on the circadian rhythm and the effect of 5-HT on the 34-kDa protein was also examined. As 5-HT causes phase shifts in the rhythm by activating adenylate cyclase to increase cAMP, forskolin and 8-benzylthioadenosine 3',5'-cyclic monophosphate mimicked the effect of 5-HT on the 34-kDa protein. We also found that 5-HT significantly increased the synthesis of the 34-kDa protein at phases when 5-HT delays or advances the phase of the rhythm but did not increase the synthesis of the 34-kDa protein at a phase when 5-HT did not phase shift. This phase-dependent effect of 5-HT on the 34-kDa protein qualitatively accounts for the phase dependence of the effect of 5-HT on the circadian rhythm. These results, when considered together with our earlier data, suggest that the synthesis of the 34-kDa protein is directly involved in the phase shift produced by 5-HT. The 34-kDa protein is worthy of future investigation as a candidate for a component of the circadian oscillator.