Aboraya, M. | Search bioRxiv

bioRxiv2026

SPACA9 Acts as a Molecular Staple Modulating Microtubule Dynamic Instability

Motile cilia rely on highly stable axonemal microtubules reinforced by microtubule inner proteins (MIPs) that form a network within their lumen, yet the functions of individual MIPs remain poorly understood. Here, we characterize the conserved MIP sperm acrosome-associated protein 9 (SPACA9), which localizes to respiratory cilia and sperm flagella. Using in vitro reconstitution assays, we show that human SPACA9 (hSPACA9) acts as a molecular staple: it stabilizes protofilaments at growing microtubule ends, and inhibits dynamic instability. Surprisingly, these effects do not confer resistance to motor-induced lattice damage, indicating that regulation of microtubule dynamics can be uncoupled from mechanical resilience. Mechanistically, we identify an unstructured C-terminal region that is sufficient for microtubule binding and recapitulates the effects on dynamics. Together, our findings reveal functional specialization among MIPs and provide a mechanistic framework for how lumenal proteins tune distinct properties of axonemal microtubules.

biophysics↗

bioRxiv2024

Allosteric modulation of protein kinase A in individuals affected by NLPD-PKA , a neurodegenerative disease in which the RIβ-L50R variant is expressed

Protein kinase A (PKA) is a crucial signaling enzyme in neurons, with its dysregulation being implicated in neurodegenerative diseases. Assembly of the PKA holoenzyme, comprising a dimer of heterodimers of regulatory (R) and catalytic (C) subunits, ensures allosteric regulation and functional specificity. Recently, we defined the RI{beta}-L50R variant as a causative mutation that triggers protein aggregation in a rare neurodegenerative disease. However, the mechanism underlying uncontrolled PKA allosteric regulation and its connection to the functional outcomes leading to clinical symptoms remain elusive. In this study, we established an in vitro model using patient-derived cells for a personalized approach and employed direct measurements of purified proteins to investigate disease mechanisms in a controlled environment. Structural analysis and circular dichroism spectroscopy revealed that cellular proteins aggregation resulted from misfolded RI{beta}-subunits, preventing holoenzyme assembly and anchoring through A Kinase Anchoring Proteins (AKAPs). While maintaining high affinity to the C subunit, the resulting RI{beta}-L50R:C heterodimer exhibits reduced cooperativity, requiring lower cAMP concentrations for dissociation. Consequently, there was an increased translocation of C-subunit into the nucleus, impacting gene expression. We successfully controlled C subunit translocation by introducing a mutation that decreased RI{beta}:C dissociation in response to elevated cAMP levels. This research thus sets the stage for developing therapeutic strategies that modulate PKA assembly and allostery, thus exerting control over the unique molecular signatures identified in the disease-associated transcriptome profile.

cell biology↗