Amino Acid Sequences of Wool Keratin IF Proteins

Summary

The proteins derived from the microfibrils of the wool fibre have amino acid sequences highly homologous with those of intermediate filament proteins, in particular the cytokeratins. Like the cytokeratins, they can be divided into two classes, type I and type II; equimolar quantities of each class are required for filament formation. This proceeds by the association of alpha-helical coiled-coil dimers to form a tetramer which is the repeat unit of the filament.

Limited chymotryptic digestion of the reduced carboxymethylated wool microfibrillar protein complex leads to excision of a tetrameric particle composed of the 1B segments of the various protein chains. As a by-product of this reaction, 1B segments are also covalently linked, probably by transpeptidation to give a mixture of dimeric species. The different combinations of 1B types in these linear dimers have been partially defined by direct isolation of the links between the 1B segments. The results of these experiments strongly suggest that the initial coiled-coil dimer is a heterodimer and places constraints on the relative disposition of the dimers within the tetramer.

The potential for disulphide bond formation between chains of microfibrillar proteins was assessed by examination of the amino acid sequences now known for four of the proteins using four simple constraints. Disulphide bond formation between chains either within or between coiled-coil dimers in the tetramer repeat is highly unlikely. However for a particular arrangement of chains in the tetramer repeat the formation of a number of disulphide bonds between overlapping segments of linearly adjacent tetramers is possible. This arrangement is also suggested by the results of the transpeptidation experiments.