Molecular survey of the phosphoserine phosphatase involved in L-serine synthesis by silkworms (Bombyx mori)

Phosphoserine phosphatase (PSP) catalyses the synthesis of l-serine via the phosphorylated pathway by facilitating the dephosphorylation of phosphoserine. A cDNA encoding PSP from the silkworm Bombyx mori (bmPSP) was isolated using reverse transcription-PCR and then sequenced. The resulting clone encoded 236 amino acids with a molecular weight of 26 150, exhibiting 14–60% sequence identity with other PSPs. The recombinant PSP was overexpressed in Escherichia coli and purified. Kinetic studies showed that bmPSP possessed activity toward l-phosphoserine, and Asp20, Asp22 and Asp204 in bmPSP were found to be critical for modulating bmPSP activity. Real-time PCR analysis provided evidence that the amount of bmpsp transcript was reduced in middle silk glands of a sericin-deficient silkworm strain. These findings revealed that bmPSP may play important roles in synthesizing one-carbon donors of l-serine, which is abundant in silk, as well as other cell metabolites in B. mori.