Hagfish Intermediate Filament Proteins: From a Natural Protein System to Functional Biomaterials

Sourcing protein-based biomaterials directly from nature presents persistent challenges in scalability, processing, and batch-to-batch variability. Recombinant engineering has focused largely on spider silks, and hagfish intermediate filament (HIF) proteins remain a comparatively unexplored alternative, with small non-repetitive subunits that have been produced full-length at high yield. This review examines the biology of naturally produced HIF protein threads from the Pacific hagfish, Eptatretus stoutii, advances in recombinant expression and purification, and their translation into functional biomaterials. We first describe the tripartite domain architecture and biochemical classification of the threonine-rich α- and γ-keratin-like subunits, and then the defensive biomechanics of HIF threads in hagfish slime, where osmolyte-regulated skeins rapidly uncoil to clog predator gills. We evaluate isolation methods from native exudate alongside thread mechanics, including a strain-induced α-to-β secondary structure transition that could account for the material’s toughness. Moving to engineered platforms, we analyze heterologous production of recombinant HIF (rHIF) in Escherichia coli and Pichia pastoris and compare purification strategies including centrifugation, size-exclusion chromatography, tangential flow filtration, and selective precipitation. We then survey rHIF biomaterial development across dry fibers, hydrogel fibers, and standalone thin films, with a focus on biomedical applications, specifically high-fidelity in vitro modeling of Bruch’s membrane.