Folding of globular proteins can be envisioned as the contraction of a random coil unfolded state toward the native state on an energy surface rough with local minima trapping frustrated species. These substructures impede productive folding and can serve as nucleation sites for aggregation reactions. However, little is known about the relationship between frustration and its underlying sequence determinants. Chemotaxis response regulator Y (CheY), a 129-amino acid bacterial protein, has been shown previously to populate an offpathway kinetic trap in the microsecond time range. The frustration has been ascribed to premature docking of the N-and C-terminal subdomains or, alternatively, to the formation of an unproductive local-in-sequence cluster of branched aliphatic side chains, isoleucine, leucine, and valine (ILV). The roles of the subdomains and ILV clusters in frustration were tested by altering the sequence connectivity using circular permutations. Surprisingly, the stability and buried surface area of the intermediate could be increased or decreased depending on the location of the termini. Comparison with the results of small-angle X-ray-scattering experiments and simulations points to the accelerated formation of a more compact, on-pathway species for the more stable intermediate. The effect of chain connectivity in modulating the structures and stabilities of the early kinetic traps in CheY is better understood in terms of the ILV cluster model. However, the subdomain model captures the requirement for an intact N-terminal domain to access the native conformation. Chain entropy and aliphatic-rich sequences play crucial roles in biasing the early events leading to frustration in the folding of CheY.CF-SAXS | Go models | CheY permutants | protein-folding intermediates H ighly denatured states of globular proteins resemble statistical random coils when examined with low-resolution techniques such as X-ray scattering (1) and hydrodynamic analyses (2). However, a higher-resolution view provided by experimental models (3-6) and simulations (7) shows that the conformational ensemble is biased toward low-contact-order (CO) structures, e.g., α-helices, β-turns, and β-hairpins, which form and melt in less than a few microseconds. During folding, these nascent structures presumably coalesce into higher-order assemblies of ever-increasing free energy until reaching the transition-state ensemble (TSE) that leads to the native conformation. From another perspective, this assembly process mediates a global collapse of the chain in an unfavorable solvent (8). Landscape theory (9) posits that, in the simplest scenario, native-like substructures appear and lead without pause to the TSE and the native conformation in an apparent two-state fashion. However, simulations have found that topological frustration, e.g., the premature formation of a substructure that impedes access to the productive TSE, can lead to the accumulation of intermediates (I) that must unfold to some extent to traverse the folding reaction coordinate...