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SSZ1 (YHR064C; synonym PDR13) encodes an atypical/non-canonical Hsp70 that functions as the Hsp70 subunit of the ribosome-associated complex (RAC) together with the J-domain protein Zuo1/zuotin, rather than as a typical standalone Hsp70.
"The literature retrieved for **SSZ1** is consistent with UniProt **P38788** from *Saccharomyces cerevisiae* (S288c): an **Hsp70-family, noncanonical/atypical Hsp70** named **Ssz1**, encoded by **SSZ1 (YHR064C; synonym PDR13)**, functioning as the Hsp70 subunit of the **ribosome-associated complex (RAC)** together with the J-domain protein **Zuo1/Zuotin**."
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RAC (Zuo1 + Ssz1) cooperates with the canonical ribosome-bound Hsp70 Ssb1/2 to form a cotranslational chaperone triad at the ribosomal tunnel exit that supports early nascent-chain handling and folding.
"In budding yeast, **RAC** is a stable heterodimeric chaperone complex at the ribosomal tunnel exit composed of **Zuo1 (Hsp40/J-domain protein)** and **Ssz1 (atypical Hsp70)**; RAC cooperates with the ribosome-bound canonical Hsp70 **Ssb1/2** to form a **functional chaperone triad** that supports early nascent-chain handling and cotranslational folding."
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Ssz1 binds nucleotide but does not detectably hydrolyze ATP, and ATP hydrolysis (and even ATP binding) can be largely dispensable in vivo, implying its primary role is not a classical ATP-driven foldase cycle.
"Unlike canonical Hsp70s, **Ssz1 binds nucleotide but is not detectably ATP-hydrolyzing** in vitro, and key parts of canonical Hsp70 functional logic are rewired: **ATP hydrolysis—and even ATP binding—can be largely dispensable in vivo** depending on the mutational context, implying Ssz1’s primary role is not a classic ATP-driven foldase cycle."
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Ssz1 has a non-canonical Hsp70 architecture (truncated/rudimentary SBD-beta, lacking the usual SBD-alpha lid and conserved linker), consistent with a specialized RAC role rather than a generic Hsp70 chaperone cycle.
"Ssz1 has a **noncanonical Hsp70 architecture**: truncated/rudimentary **SBD-β**, lacks the usual **SBD-α lid** and conserved linker, and uses an extended linker intertwined with the **Zuo1 N terminus** to stabilize RAC"
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RAC is a ribosome-associated system localized near the 60S subunit tunnel-exit region, with Zuo1 anchoring the complex at the ribosome and Ssz1 tethered through Zuo1; RAC occupancy is roughly 0.3-0.5 per ribosome.
"RAC is a **ribosome-associated** system localized near the **60S subunit tunnel exit region**, with Zuo1 anchoring RAC at the ribosome and Ssz1 tethered through Zuo1."
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Mechanistically, in a relay model Ssz1's rudimentary substrate-binding features support transient, low-affinity interactions with emerging nascent chains, helping channel substrates toward productive Ssb capture.
"in the relay model, Ssz1’s rudimentary substrate-binding features support **transient, low-affinity interactions** with emerging nascent chains that help channel substrates toward productive Ssb capture."
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Loss of SSZ1 causes slow growth and cold sensitivity, phenotypes shared with loss of Zuo1 or Ssb1/2, consistent with action in a common ribosome-associated chaperone pathway.
"Loss of SSZ1 causes **slow growth and cold sensitivity**, phenotypes shared with loss of Zuo1 or Ssb1/2, consistent with action in a common ribosome-associated chaperone pathway."
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Ssz1 contributes to accurate translation; RAC/Ssz1 defects produce paromomycin/aminoglycoside sensitivity and translational-fidelity phenotypes that are partly separable from general growth/cold-sensitivity phenotypes.
"Ssz1 contributes to **accurate translation**; RAC/Ssz1 defects produce **paromomycin/aminoglycoside sensitivity** and translational-fidelity phenotypes."
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RAC antagonizes prion formation (e.g., effects on [PSI+]) through its role in cotranslational folding and nascent-chain quality control; this is a downstream proteostasis consequence of its chaperoning role.
"RAC has been implicated in **antagonizing prion formation** (e.g., effects on [PSI+]) through its role in cotranslational folding and nascent-chain quality control"
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The PDR13 synonym reflects a historical link to pleiotropic drug resistance, but the strongest evidence supports Ssz1's primary role as a ribosome-associated cotranslational chaperone component rather than a transporter or enzyme; PDR phenotypes are best interpreted as indirect proteostasis/translational effects.
"the strongest mechanistic evidence in this evidence set supports Ssz1’s **primary role as a ribosome-associated chaperone component** rather than a transporter or enzyme"