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 α-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.