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DISCUSSION 

 W. D. McElroy, A. J. Hodge, W. H. Johnson, C. L. Prosser, F. G. Sherman 



Dr. McElroy (Johns Hopkins University): Could you describe the ATP 

 eflFect in greater detail? Does inorganic pyrophosphate or triphosphate work? 

 What does the addition of Mg+ + do to this process? 



Dr. Hodge: ATP is not unique in its abiHty to produce SLS-type aggre- 

 gates from acid solutions of collagen; ITF and nucleic acids will also yield 

 segment-type structures under appropriate conditions. In fact, the SLS form 

 was first discovered by Schmitt et at. (1953) when they dialyzed a phosphate 

 extract of skin collagen (pH = 8, /t = 0.4) against citrate buffer (pH 4, 

 IX = 0.2). Such extracts show strong absorption at about 2600 A. The sodium 

 salts of ATP and ITP do not work, nor does inorganic pyrophosphate or tri- 

 phosphate. These results suggest that the binding of ATP to the TC macro- 

 molecules modifies the charge distribution in a way such that (a) the iso- 

 electric point of the TC is lowered and (b) the parallel, in-register SLS-type 

 of packing is favored over the normal staggered configuration of native-type 

 collagen. The presence of appreciable concentrations of salts prevents SLS 

 formation, and SLS-type structures are redissolved if salt is added, suggesting 

 that the linkages involved are electrostatic and relatively weak. I know of no 

 special effect of Mg++ on the SLS system, other than its contribution to the 

 ionic strength of the medium. 



Dr. Johnson (University of Illinois): Do you think that spatial separation 

 of charged sequences along the molecule would account for periodicities seen 

 in electron micrographs of precipitated preparations of other proteins, such as 

 paramyosin, tropomyosin, and light meromyosin? We have evidence for the 

 presence of polyglutamic acid along the paramyosin molecule. Could =this 

 account for periodicity in paramyosin? 



Dr. Hodge: Yes. The band patterns observed in the various ordered aggre- 

 gation kates of proteins such as paramyosin, tropomyosin, and light mero- 

 myosin are almost certainly due to a precise distribution of "clusters" of polar 

 residues separated axially by regions of non-polar residues. As is the case for 

 collagen, it is the precise spatial separation of these polar regions and their 



