INTERMEDIATE FILAMENT STRUCTURE - HARD ALPHA-KERATIN

Structurally there are four classes of intermediate filaments (IF) wit h distinct but closely related axial organisations. One of these, hard alpha-keratin IF, has been studied to clarify several apparently exce ptional features which include the number of molecules in the IF cross -section and the mode by which the axial organisation of its constitue nt molecules is stabilised. Using the dark-field mode of the STEM at t he Brookhaven National Laboratory (USA) mass measurements were obtaine d from unstained IF isolated from hair keratin. The data thus obtained show that the number of chains in cross-section is about 30 (+/-3: st andard deviation) and is very similar to the numbers determined in pre vious STEM experiments for the dominant filament type in other classes of IF (about 32). Furthermore, re-analysis of the low-angle equatoria l X-ray diffraction pattern reveals, in contrast to earlier work, solu tions that are compatible with the number of chains in cross-section i ndicated by the STEM data. The absence of the head-to-tail overlap bet ween parallel molecules characteristic of most IF may be compensated i n hard alpha-keratin by a network of intermolecular disulfide bonds. I t is concluded that native IF of hard alpha-keratin and desmin/vimenti n - and probably many other kinds of IF as well - contain about 32 cha ins in cross-section, and that the axial structures of these various k inds of IF differ in small but significant ways, while generally obser ving the same basic modes of aggregation. (C) 1997 Elsevier Science B. V.