PIH1D2

PIH1D2 is an adaptor subunit of the R2SP quaternary chaperone complex, in which it associates with SPAG1 and the AAA+ ATPases RUVBL1/RUVBL2 to promote the assembly of multi-subunit protein clients (PMID:29844425). R2SP is enriched in testis and is required for the stability and assembly of liprin-α2 complexes, functioning as a temperature-sensitive quaternary protein folding machine (PMID:29844425). PIH1D2 is also specifically required for the assembly of distinct axonemal dynein subtypes in motile cilia and flagella; loss of pih1d2 in zebrafish results in absence of specific dynein arms and defective sperm motility (PMID:29741156).

1. 2018 High

   Two independent studies established that PIH1D2 functions within a novel R2TP-like chaperone complex (R2SP) and is required for assembly of specific multi-subunit clients — liprin-α2 complexes and axonemal dynein subtypes — resolving its molecular context and biological roles in quaternary protein folding and motile cilia function.

   Evidence Co-IP/pulldown interaction mapping and client expression assays in human cells (PMID:29844425); zebrafish pih1d2 genetic knockout with cryo-electron tomography and sperm motility assays (PMID:29741156)

   PMID:29741156 PMID:29844425

   Open questions at the time

   • How PIH1D2 selects or recognizes specific dynein subtypes versus other potential clients is unknown
   • Whether PIH1D2 loss causes motile ciliopathy phenotypes beyond spermatozoa (e.g. airway cilia) has not been tested in mammals
   • The direct contacts between PIH1D2 and client proteins have not been mapped
2. 2019 Medium

   Biochemical dissection of the R2SP complex revealed that the co-chaperone recruitment and ATPase regulatory activities reside in SPAG1 rather than PIH1D2, clarifying the division of labor within the complex.

   Evidence ITC, NMR spectroscopy, molecular dynamics simulations, and in vitro hydrolysis assays on SPAG1 TPR domains and RUVBL1/2

   PMID:31118266

   Open questions at the time

   • PIH1D2's specific biochemical contribution to R2SP function (beyond scaffold membership) was not directly assayed
   • Whether PIH1D2 modulates HSP70/HSP90 recruitment or RUVBL ATPase activity allosterically is untested
3. 2025 Medium

   Structural analysis of the intact human R2SP complex showed an architecture related to but distinct from canonical R2TP, with differences in RUVBL1/2 ATPase activity and in how SPAG1 and PIH1D2 bind the RUVBL ring, providing the first three-dimensional model of the holocomplex.

   Evidence NMR, structural mass spectrometry, cryo-EM, and ATPase assays (preprint)

   PMID:bio_10.1101_2025.01.27.635100

   Open questions at the time

   • Preprint not yet peer-reviewed
   • Structure of R2SP bound to a client protein has not been determined
   • Functional consequences of the distinct ATPase properties relative to R2TP are not established

• The mechanism by which PIH1D2 directly recognizes and delivers specific protein clients to the RUVBL1/2 ATPase machinery, and whether PIH1D2 has roles beyond testis and motile cilia, remain unresolved.
• No client-binding interface on PIH1D2 has been defined
• PIH1D2 function in somatic tissues outside testis is unexplored
• Whether PIH1D2 mutations cause human disease (e.g. primary ciliary dyskinesia or male infertility) has not been investigated