The core protease (CP) subcomplex of the 26S proteasome houses the proteolytic active sites and assumes a barrel shape comprised of four co-axially stacked heptameric rings formed by structurally related α- and β-subunits. CP biogenesis typically begins with the assembly of the α-ring, which then provides a template for β-subunit integration. In eukaryotes, α-ring assembly is partially mediated by two hetero-dimeric chaperones, termed Pba1–Pba2 (Add66) and Pba3–Pba4 (also known as Irc25–Poc4) in yeast. Pba1–Pba2 initially promotes orderly recruitment of the α-subunits through interactions between their C-terminal HbYX or HbF motifs and pockets at the α₅–α₆ and α₆–α₇ interfaces. Here, we identified PBAC5 as a fifth α-ring assembly chaperone in Arabidopsis that directly binds the Pba1 homolog PBAC1 to form a trimeric PBAC5–PBAC1–PBAC2 complex. PBAC5 harbors a HbYX motif that docks with a pocket between the α₄ and α₅ subunits during α-ring construction. Arabidopsis lacking PBAC5, PBAC1 and/or PBAC2 are hypersensitive to proteotoxic, salt and osmotic stresses, and display proteasome assembly defects. Remarkably, whereas PBAC5 is evolutionarily conserved among plants, sequence relatives are also dispersed within other kingdoms, including a scattered array of fungal, metazoan and oomycete species.

26S 蛋白酶体的核心蛋白酶 (CP) 亚复合物包含蛋白水解活性位点，并呈桶状，由四个同轴堆叠的七聚环组成，这些七聚环由结构相关的 α 和 β 亚基形成。CP 生物发生通常始于 α 环的组装，然后为 β 亚基整合提供模板。在真核生物中，α 环组装部分由两个异二聚体伴侣介导，在酵母中称为 Pba1-Pba2 (Add66) 和 Pba3-Pba4（也称为 Irc25-Poc4）。_{Pba1–Pba2 最初通过其 C 端 HbYX 或 HbF 基序与 α 5} –α ₆和 α ₆ –α ₇界面处的口袋之间的相互作用促进 α 亚基的有序募集。在这里，我们确定 PBAC5 是拟南芥中的第五个 α 环组装分子伴侣，它直接结合 Pba1 同源物 PBAC1 形成三聚体 PBAC5-PBAC1-PBAC2 复合物。PBAC5 包含 HbYX 基序，在 α 环构建过程中与 α ₄和 α ₅亚基之间的口袋对接。缺乏 PBAC5、PBAC1 和/或 PBAC2 的拟南芥对蛋白毒性、盐和渗透胁迫高度敏感，并表现出蛋白酶体组装缺陷。值得注意的是，虽然 PBAC5 在植物中进化上是保守的，但序列亲属也分散在其他界中，包括分散的真菌、后生动物和卵菌物种。