Maturation of hagfish gland thread cells: Composition and characterization of intermediate filament polypeptides

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Abstract
Previous studies with the hagfish, a primitive vertebrate, have shown that the gland thread cells (GTCs) each contain a single thread (∼60 cm long in average‐sized cells) in the form of a concisely coiled cytoskeletal entity destined for export by holocrine secretion. The thread in relatively immature GTCs consists almost entirely of intermediate filaments (IFs) bundled in parallel alignment with far fewer microtubules (MTs). The three thread polypeptides described earlier (α, basic; β acidic; γ, most acidic; each with a Mr of 63–64 kD) are now further evaluated with respect to in vitro assembly, cross‐reactivity with IF polypeptides from higher vertebrates, and peptide sequence homology with known IF polypeptides. The overall results mainly suggest that the hagfish polypeptides are keratinlike substances but lamins or a new type of IF is not ruled out. However, cross‐reactivity is weak with mammalian keratins; the 8–11‐nm filaments formed from mixtures of α and γ in vitro are generally linear rather than the curvilinear structures usually formed by keratin and nonkeratin IFs; and mixtures of α and β tend to yield 9–12‐nm granules or granular strings. Polypeptide analyses on GTCs segregated on the basis of maturational stage show a progressive increase in β/γ values which correlates with cell maturation, but the α/(β+γ) ratios remain near 1. Inasmuch as β and γ have many similar properties, the documented increase in the amount of the β component in aging GTCs might in part be the result of a failure in a posttranslational modification system and may contribute to the ultrastructural changes that accompany thread maturation in preparation for holocrine secretion and subsequent modulation of the viscoelastic properties of mucus.