Phytolacca americana lectin (Pa-2; pokeweed mitogen): an intrinsically unordered protein and its conversion into partial order at low pH.

This is the first report of its kind that well demonstrates that a lectin from Phytolacca americana [Pa-2 (P. americana lectin-2)] can also be intrinsically unordered, based on the results obtained by CD, tryptophan fluorescence, ANS (8-anilinonaphthalene-1-sulfonic acid) binding, acrylamide quenching, DLS (dynamic light scattering) and its amino acid composition database analyses. Pa-2 is an acidic monomeric lectin and acquires random coil conformation at neutral pH without any regular secondary structure. As confirmed by different spectroscopic techniques, on lowering the pH, some secondary structures, predominantly alpha-helices, are detected by far-UV CD that adopt a marginally stable partially folded collapsed conformation possessing the characteristics of a premolten globule state. It is in accordance with coil-helix transition that is commonly observed when these intrinsically unordered proteins interact with their partner molecules in vivo.