Recognition of signal peptide by protein translocation machinery in middle silk gland of silkworm Bombyx mori.

To investigate the functions of signal peptide in protein secretion in the middle silk gland of silkworm Bombyx mori, a series of recombinant Autographa californica multiple nucleopolyhedroviruses containing enhanced green fluorescent protein (egfp) gene, led by sericin-1 promoter and mutated signal peptide coding sequences, were constructed by region-deletions or single amino acid residue deletions. The recombinant Autographa californica multiple nucleopolyhedroviruses were injected into the hemocoele of newly ecdysed fifth-instar silkworm larvae. The expression and secretion of EGFP in the middle silk gland were examined by fluorescence microscopy and Western blot analysis. Results showed that even with a large part (up to 14 amino acid residues) of the ser-1 signal peptide deleted, the expressed EGFP could still be secreted into the cavity of the silk gland. Western blot analysis showed that shortening of the signal peptide from the C-terminal suppressed the maturation of pro-EGFP to EGFP. When 8 amino acid residues were deleted from the C-terminal of the signal peptide (mutant 13 aa), the secretion of EGFP was incomplete, implicating the importance of proper coupling of the h-region and c-region. The deletion of amino acid residue(s) in the h-region did not affect the secretion of EGFP, indicating that the recognition of signal peptide by translocation machinery was mainly by a structural domain, but not by special amino acid residue(s). Furthermore, the deletion of Arg2 or replacement with Asp in the n-region of the signal peptide did not influence secretion of EGFP, suggesting that a positive charge is not crucial.