HOLFORD, K.C.*; BORST, D.W.: Phosphorylation regulates farnesoic acid O-methyl transferase activity in the mandibular organ of the lobster, Homarus americanus.

Synthesis of methyl farnesoate (MF) by the lobster mandibular organ (MO) is regulated by a neuroendocrine peptide (MO-inhibiting hormone; MOIH) secreted by the sinus gland (SG), a neurosecretory organ in the eyestalk. MOIH regulates MF production by reducing the activity of methyl transferase (MeT), the final enzyme in MF biosynthesis. Eyestalk ablation (ESA) increases MeT activity, MF hemolymph levels, and MeT transcript levels. Treatment of ESA lobsters with SG extract reduces MeT activity and lowers circulating MF levels after 4 hours, but has no effect on transcript levels. These data suggest that the SG extract affects enzyme activity through modification of MeT. Therefore, we tested the hypothesis that MOIH treatment alters MeT activity by changing the phosphorylation state of the enzyme. MO preparations from ESA lobsters (with high levels of MeT activity) were treated with alkaline phosphatase. This treatment reduced MeT activity by 53% (SE=4.0, n=4, P=0.03) compared to untreated controls. The phosphatase activity of MO membranes was quantified by measuring p-nitrophenylphosphate hydrolysis. Membranes from ESA animals that had been treated with SG extracts had higher levels of phosphatase activity (28% higher) than ESA animals treated with saline (P=0.04, n=3). Preliminary evidence indicates that SGE-treated membrane preparations can reduce MeT activity. In summary, these data are consistent with the model that MOIH stimulates a protein phosphatase that inactivates MeT through dephosphorylation. (Supported by Purdue Research Foundation summer grant to KCH and NIH R15 HD37953-01 to DWB).