Hagfish slime gland thread cells. II. Isolation and characterization of intermediate filament components associated with the thread.

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Abstract
The slime glands of hagfish have 2 major cell types, gland thread cells (GTC) and gland mucous cells (GMC), both of which upon contact with water contribute to the formation of an abundant quantity of viscous mucus. Each ellipsoidal thread cell normally contains a single tapered thread which is uniquely coiled into a space-saving conformation and occupies most of the cell volume. The developing thread consists mainly of intermediate filaments (IF) aligned in parallel not only to one another but also to a far fewer number of interspersed microtubules. Urea extracts of GTC were purified and characterized to establish the properties of thread components. One major (.alpha.) and 2 minor (.beta., .gamma.) components prepared by anion exchange chromatography have similar apparent MW of 63,500 .+-. 500 daltons but different isoelectric pH values (.alpha., 7.56; .beta., 5.67; .gamma., 5.31). Although the amino acid content of .alpha. differed significantly from .beta. and .gamma., each of the 3 was highest in Gly, relatively high in Glx, Ser, Thr, Asx, Ala, Val and Leu, and relatively low in Cys/2 and Trp. The amino acid compositions of .beta. and .gamma. were very similar, and only .beta. showed evidence of carbohydrate. The threonine content of the .alpha. component was higher than reported for IF of different origin, and the high content of hydroxyamino acids (18, 19 residues per 100) in .alpha., .beta., and .gamma. was approached only by several IF polypeptides from human or bovine epidermal keratins. Mixtures of the purified components formed 9-11-nm filaments in vitro. The hagfish thread cell is a rich source of IF, which have a structure that facilitates formation of macrofibrils within the cell.