Authors:

Julia Gordan

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About Paper:

Cellular proteostasis depends on molecular chaperones that recognize unstable protein intermediates, including "orphan" proteins that temporarily lack their normal binding partners and are therefore vulnerable to misfolding, aberrant condensation, and aggregation. In Saccharomyces cerevisiae, the Hsp40 co-chaperone Sis' provides an early defense by capturing orphan-prone clients and promoting productive folding or transfer to the Hsp70 system. We sought to define structural principles governing Sis1-client recognition across diverse substrates and to test whether Sis1 preferentially engages surfaces that overlap with clients' native partner interfaces. Using AlphaFold Multimer, we modeled Sis1 in complex with candidates spanning distinct cellular pathways and aggregation/condensation liabilities (Cct3, Rps3, Tub2, Sup35, Rngq1, Tub1, Act1, Ubc9, and Rpl26a) [1]. We evaluated predicted complexes for consistent docking geometry and interface plausibility, then compared binding modes across clients to identify recurring contact regions and client-specific determinants [2]. The models indicate substrate-specific binding modes rather than a single universal interface, consistent with conformational adaptability that accommodates heterogeneous client surfaces [3]. Despite this diversity, a recurring pattern emerged: Sis1 frequently occupies an interface that coincides with the substrate's native binding-partner surface, consistent with a placeholder mechanism that shields assembly-critical regions during orphan windows until appropriate partners become available. Several substrates in the panel are known to form condensates when unchaperoned, aligning predicted Sis1 engagement with proteostasis-relevant risk states. Together, these structural predictions support a model in which Sis' acts as a first-line anti-aggregation gatekeeper by flexibly recognizing multiple clients while preferentially masking partner-facing interfaces. The resulting interface map provides a practical foundation for targeted mutagenesis and future engineering of Sis1 variants with tuned client specificity to mitigate proteotoxic stress in experimental disease and aggregation models.

Source:

University of Chicago

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Co-authors:

Julia Gordan