Abstract: Research of the protein composition of different parts of the middle silk gland (MSG) cells of the fifth instar larvae of silkworm （Bombyx mori L.）are useful to find the functional protein concerned with the composition and secretion of the sericin protein. Using the protein two dimensional electrophoresis (2D-PAGE) and the image analysis technology, a significant differences of the protein composition among the anterior, middle and posterior parts of the MSG cells of silkworm were found. Twenty specific proteins that expressed only in anterior part of the MSG cells were likely to relate with the composition and secretion of 4 kinds of sericin proteins, which are Ser2A, Ser2B, S4 and S5. Twenty-two specific proteins that expressed only in middle part of the MSG cells were likely to relate with the composition and secretion of 4 kinds of sericin proteins, which are Ser1B, Ser1C, Ser1D and S3. Twenty-seven specific proteins that expressed only in posterior part of the MSG cells were likely to relate with the composition and secretion of 2 kinds of sericin proteins, which are Ser1A and S3. Moreover, fifty-one specific proteins which expressed in middle and posterior, but not in anterior part of MSG cells and twenty proteins whose volume in middle and posterior part of MSG cells were higher than that in anterior part likely participated in the synthesis of the sericin protein of the corresponding parts.

Key words: Silkworm (Bombyx mori L.), Silk gland, 2D-PAGE, Image analysis, Sericin protein