Identification of four small molecular mass proteins in the silk of Bombyxmori

This paper describes cDNAs of four small-size proteins that occur in the co coon silk of Bombyx mori. Two of them (9.9 and 10.3 kDa), which have the N- terminal sequences and the spacing of a few amino acids at C-termini simila r to the seroin of Galleria mellonella, are called seroin 1 and seroin 2. T he corresponding genes are expressed in the middle, and to a small extent a lso in the posterior silk gland sections. The seroin I and less conspicuous ly the seroin 2 mRNAs accumulate in the course of the last larval instar to a maximum in postspinning larvae. Two other proteins (6 kDa and 4.7 kDa) o f B. mori cocoons were identified as a typical Kunitz-type and a somewhat u nusual Kazal-type proteinase inhibitors, and named BmSPI 1 and BmSPI 2, res pectively. Their genes are expressed in the middle, and the BmSPI 1 gene sl ightly also in the posterior silk gland sections. The expression ensues a f ew days after the last larval ecdysis and increases until the cocoon spinni ng. Post-spinning larvae still contain high amounts of the BmSPI 1 but no B mSPI 2 transcripts. It is assumed that seroins and proteinase inhibitors ar e involved in cocoon protection against predators and microbes.