Cell Constitution 



47 



Large Periods in Protein Fibers. Small- 

 angle x-ray data have shown that some 

 fibrous proteins are characterized by large 

 axial and transverse periods (Table 1). Al- 

 though it has been suggested in several cases 

 that the axial period represents the length 

 of the molecule, this has not yet been proven. 

 Indeed, in some cases (collagen, paramyosin) 

 it has been possible, by solution and appro- 

 priate reconstitution, to produce fibrils which 

 manifest an axial repeat period several times 

 that of the native fibers. It is of course pos- 

 sible that, in the native state, the chains are 

 coiled and that, in passing from the normal 

 length of the period to the "long-spacing" 

 form, the chain molecules become fully ex- 

 tended and then manifest the long-spacing 

 period. 



From the x-ray data, it has been possible 

 also to deduce something about the intra- 

 period fine structure. In the case of collagen, 

 Bear ('52) suggests that the neighboring 

 chains form parallel, ordered regions alter- 

 nating with more poorly ordered regions. 

 These regions have precise position within 

 the main repeat period, giving rise to intra- 

 period bands as seen in the EM. The rela- 

 tive order or disorder is thought to depend 

 upon the length of the side chains and the 

 perfection of packing of these side chains 

 between neighboring polypeptide chains. 

 This would imply that there is a fairly pre- 

 cise sequence of amino acid types, if not of 

 amino acids themselves, along the chain 

 molecules, a view for which there is analyt- 

 ical evidence in the case of globular proteins 

 (Sanger, '52). 



It seems possible that characteristic, long 

 axial repeating periods may be discovered in 

 many other fibrous proteins thus far not ade- 

 quately investigated. This is of considerable 

 importance because such periods provide 

 molecular "fingerprints" by which proteins 

 may be identified in tissues and in various 

 kinds of preparations and give information 

 about molecular structure. 



Classification of Fibrous Proteins. Astbury 

 early pointed out that the wide-angle, alpha, 

 patterns of certain proteins are closely simi- 

 lar. Keratin, myosin, epidermin and fibrin 

 are grouped together as the "KMEF class." 

 A different large-angle pattern characterizes 

 a group of proteins called by Astbury the 

 "collagen class." The latter group includes 

 the collagens of vertebrates and invertebrates 

 and a number of proteins from forms as low 

 phylogenetically as the Porifera. Astbury 

 suggested that the polypeptide chains of 

 members of the collagen class are not folded 



upon themselves as are those of the KMEF 

 class but are essentially fully extended. Re- 

 cent analyses (Bear, '52) indicates that 

 chains of the members of the collagen class 

 probably have a helical configuration as do 

 members of the KMEF class although the 

 geometry of the helix may differ in detail. 

 It is interesting to note that, despite strik- 

 ing differences in chemical composition, 

 various members of the collagen class which 



Table 1. Large Repeating Periods in Fibrous 

 Proteins 



REPEATING PERIOD 



have been carefully studied manifest not 

 only the same large-angle pattern but the 

 same axial long period (about 640 A) as 

 well. Each member of the KMEF class has a 

 different, characteristic axial long period (see 

 Table 1). The meaning of these facts will 

 probably not be fully understood until much 

 more is known about the chemical composi- 

 tion and molecular configuration of these 

 proteins. 



ELECTRON MICROSCOPE ANALYSIS 



The hierarchies of fiber sizes are well 

 demonstrated in EM studies. It has long 

 been known that macroscopic fibers, such as 

 muscle or tendon, having widths from 10 to 

 lOOyit, are composed of thinner fibrils which 

 may be at or below the limit of resolution of 

 the light microscope (Heidenhain). The elec- 

 tron microscope has shown that these fibrils 

 (whose presence was deduced from their 

 positive form birefringence) have widths 

 ranging from several hundred to several 



