Genetic heterogeneity of collagens.

The term collagen has recently been expanded to include at least 7 genetically distinct structural elements of mammalian connective tissues. It is assumed that differences in the primary structure specify the interactions of these different collagens with one another and with noncollagen connective tissue elements. The specific biomechanical properties of individual connective tissues result from the relative content of the different collagen types. Four major classes of collagens can be defined on the basis of compositional and structural characteristics. The first class includes collagens Type I, II, and III which form classically described compact banded film structures resulting from crosslink stabilized side-by-side interactions of the triple helical structural domains. Type IV, or basement membrane collagen, comprises the second class. These molecules form open fiber structures by disulfide stabilized end-to-end interactions of the nonhelical amino and carboxyterminal structural domains. The third class of collagens contains the Type V collagens and the molecules containing the E and F chains found in cartilage. These molecules may interact by a combination of side-by-side and end-to-end aggregation. Still a fourth class is suggested by recent descriptions of several collagens which appear to contain extensive regions of unstable triple helix within the triple helical structural domain. It has been suggested that these collagens may serve as links between collagen and noncollagen structural elements. These concepts suggest that the specificity of interactions directed by the different collagen types result from (1) the extent of removal of the nonhelical collagen domains and (2) the integrity of the triple helical structure within the triple helical domain. These 2 properties of the different collagens are directly specified by genetically determined amino acid sequence differences.