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Templated aggregation of na\u00efve tau to seeding-competent fibrils that proceed from cell to cell is a key driver of prion-like progression of tauopathies. This study tests the hypothesis that tau, an intrinsically disordered protein (IDP), achieves in-register stacking to form seed-competent fibrils by a pinning action of tau to each other and\/or the seed surface via a single dominant hotspot to avoid mismatch in tau stacking to fibrils. Structured solvation water has been proposed to be a signature of such hotspots at both the tau fibril-end surface and soluble tau monomers. Although jR2R3-P301L tau exhibits a heterogeneous hydration landscape in its intrinsically disordered state, with enhanced water structuring near the P301L mutation site, it is unclear whether a localized hotspot exists at the fibril end surface and surface water facilitates the initial contacts in templated aggregation. Using rapid\n                  <jats:sup>1<\/jats:sup>\n                  H-\n                  <jats:sup>15<\/jats:sup>\n                  N SOFAST-HMQC NMR to track seed-induced aggregation of jR2R3-P301L in real time, complemented by molecular dynamics simulation of fibril surface hydration, we identify a residue-specific pinning hotspot that is prone to dewetting followed by sequential folding and incorporation of the remaining segment in a two-step dock-and-lock process. Site-specific spin labeling further demonstrates that blocking this pinning hotspot disrupts templated aggregation, leading to shorter fibrils. The identification of a dominant pinning site will facilitate the rational design of binders to effectively disrupt fibril extension or serve as diagnostic or therapeutic strategies.\n                <\/jats:p>\n                <jats:sec>\n                  <jats:title>Significance Statement<\/jats:title>\n                  <jats:p>Tau proteins must align and stack precisely with existing fibril ends to propagate pathology, yet the molecular signature that initiates and ensures this in-register alignment has been unclear. This study shows recruitment begins at a single, structurally well-defined contact site on the fibril surface that is enriched in release-prone hydration water. These findings reveal that water-release-prone hotspots, rather than the well-known amyloid-core regions alone, govern the initial steps in templating seeding and can hence be blocked, opening a new path for identifying therapeutic targets that could slow the progression of tau-related diseases.<\/jats:p>\n                <\/jats:sec>","DOI":"10.64898\/2026.08.04.742883","type":"posted-content","created":{"date-parts":[[2026,8,10]],"date-time":"2026-08-10T04:30:11Z","timestamp":1786336211000},"source":"Crossref","is-referenced-by-count":1,"title":["A water-pinning hotspot drives templated tau aggregation"],"prefix":"10.64898","author":[{"ORCID":"https:\/\/orcid.org\/0009-0006-0924-3617","authenticated-orcid":false,"given":"Chung-Ta","family":"Han","sequence":"first","affiliation":[{"name":"Department of Chemistry, Northwestern University"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0002-9149-5936","authenticated-orcid":false,"given":"Karen","family":"Tsay","sequence":"additional","affiliation":[{"name":"Department of Chemistry and Biochemistry, University of California, Santa Barbara"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-1162-8798","authenticated-orcid":false,"given":"Samuel","family":"Lobo","sequence":"additional","affiliation":[{"name":"Department of Chemical Engineering, University of California, Santa Barbara"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Saeed","family":"Najafi","sequence":"additional","affiliation":[{"name":"Department of Chemistry and Biochemistry, University of California, Santa Barbara"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Kieran","family":"Monahan","sequence":"additional","affiliation":[{"name":"Department of Chemistry, Northwestern University"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Erica","family":"Hong","sequence":"additional","affiliation":[{"name":"Department of Chemistry, Northwestern University"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-0439-1534","authenticated-orcid":false,"given":"M. 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