This is a renewal proposal (years 5-10) to continue studies of the folding and assembly mechanisms in multisubunit proteins. Specifically, the questions to be addressed are: (1) What are the important structural features of the transient intermediates and intervening transition states observed during the folding of specific representative multimeric protein? (2) Does pre-formed structure in the monomer affect the folding kinetics of the dimer? (3) Do partially folded associated species guide the refolding reaction to the native dimer (trimer)? A combination of biophysical techniques spanning mutagenesis, hydrogen-deuterium exchange, NMR, Mass Spectrometry and rapid-mixing optical techniques will be used to gain information on the time-dependent structure formation and association reactions during the folding of three distinct systems: (a) coiled-coils, (b) the trp-repressor and (c) the trimeric beta-helix proteins.