Possible evolutionary links between immunoglobulin light chains and other proteins involved in amyloidosis

With limited exceptions, proteins that account for the amyloidoses appear to be evolutionarily unrelated. Transthyretin is classified as having an "immunoglobulin-like" fold as found in light chain variable and constant domains. Thus, these amyloidogenic proteins have significant conformational similarity. In the absence of primary structure similarity sufficient to justify an inference of an evolutionary relationship, transthyretin is considered an analog of immunoglobulin domains having accrued the immunoglobulin-like fold by some form of convergent evolution of structure. Improvements in sequence comparison tools and strategies, coupled with recent logarithmic increases in the availability of primary structure data, now make it possible to suggest that transthyretin and immunoglobulins may have a common evolutionary origin. In addition, lactadherin, the medin fragment of which accounts for the most common form of human amyloid, also appears to be evolutionarily linked to transthyretin and immunoglobulins.