Core component of the silk filament; a strong, insoluble and chemically inert fiber. Silk fibroin elementary unit consists in a disulfide-linked heavy and light chain and a p25 glycoprotein in molar ratios of 6:6:1. This results in a complex of approximately 2.3 MDa. Produced exclusively in the posterior (PSG) section of silk glands, which are essentially modified salivary glands. Composed of antiparallel beta sheets. The strands of the beta sheets run parallel to the fiber axis. Long stretches of silk fibroin are composed of microcrystalline arrays of (-Gly-Ser-Gly-Ala-Gly-Ala-)n interrupted by regions containing bulkier residues. The fiber is composed of microcrystalline arrays alternating with amorphous regions. The interchain disulfide bridge is essential for the intracellular transport and secretion of fibroin.