many cellular proteins are  disordered 
 in isolation . 
 a subset of these intrinsically disordered proteins ( idps ) can , upon 
 binding another molecule , fold to a well - defined three - dimensional 
 structure . in the structurally heterogeneous , unbound ensemble of 
 these idps , conformations are likely to exist that , in part , resemble 
 the final bound form . it has been suggested that these conformations , 
 displaying 
 residual structure , could be important 
 for the mechanism of such coupled folding and binding reactions . 
 puma , 
 of the bcl-2 family , is an idp in isolation but will form a single , 
 contiguous -helix upon binding the folded protein mcl-1 . using 
 the helix - breaking residue proline , we systematically target each 
 potential turn of helix of unbound puma and assess the binding to 
 mcl-1 using time - resolved stopped - flow techniques . all proline - containing 
 mutants bound , and although binding was weaker than the wild - type 
 protein , association rate constants were largely unaffected . 
 we conclude 
 that population of particular residual structure , containing a specific 
 helical turn , is neither required for the binding nor for fast association 
 of puma and mcl-1 .