Molecular density-accelerated binding-site maturation underlies CENP-T-dependent kinetochore assembly
Macromolecular assembly depends on tightly regulated pairwise binding interactions that are selectively favored at assembly sites while being disfavored in the soluble phase. This selective control can arise due to molecular density-enhanced binding, as recently found for the kinetochore scaffold protein CENP-T. When clustered, CENP-T recruits markedly more Ndc80 complexes than its monomeric counterpart, but the underlying molecular basis remains elusive. Here, we use quantitative in vitro assays to reveal two distinct mechanisms driving this behavior. First, Ndc80 binding to CENP-T is a two-step process: initially, Ndc80 molecules rapidly associate and dissociate from disordered N-terminal binding sites on CENP-T. Over time, these sites undergo maturation, resulting in stronger Ndc80 retention. Second, we find that this maturation transition is regulated by a kinetic barrier that is sensitive to the molecular environment. In the soluble phase, binding site maturation is slow, but within CENP-T clusters, this process is markedly accelerated. Notably, the two Ndc80 binding sites in human CENP-T exhibit distinct maturation rates and environmental sensitivities, which correlate with their different amino-acid content and predicted binding conformations. This clustering-induced maturation is evident in dividing human cells, suggesting a distinct regulatory entry point for controlling kinetochore assembly. We propose that the tunable acceleration of binding site maturation by molecular crowding may represent a general mechanism for promoting the formation of macromolecular structures. Significance StatementA distinctive mechanism of protein-protein interaction underpins the assembly of kinetochores, which is critical for human cell division. During mitosis, the Ndc80 complex must bind tightly to the unstructured N-terminus of its receptor, CENP-T, which is densely clustered at kinetochores. Using single-molecule in vitro assays, we show that Ndc80 binding is mediated by an initially unstable yet tunable interface. The high molecular density of CENP-T at the kinetochores accelerates the maturation of this binding interface, favoring the formation of stable complexes within the kinetochore structure, rather than in the soluble phase. This environment-driven modulation of binding site maturation may represent a key regulatory mechanism for ensuring strong and specific interactions during the assembly of macromolecular complexes such as kinetochores.