Signal recognition particle 9 kDa protein

SRP9 (Signal recognition particle 9 kDa protein) is a small ribonucleoprotein component of the signal recognition particle (SRP) complex. Together with SRP14 and the Alu RNA domain, it forms the elongation arrest domain of SRP and is essential for SRP RNA binding (UniProt: P49458). The SRP9–SRP14 heterodimer plays a central role in cotranslational targeting of secretory and membrane proteins to the endoplasmic reticulum during protein synthesis.

SRP9 functions broadly in the secretory pathway and protein quality control mechanisms common to all cell types, including neurons. Its involvement in ER-directed protein synthesis connects it to processes required for normal neuronal function and homeostasis.

In Alzheimer's disease, SRP9 is upregulated in post-mortem AD brain tissue relative to age-matched controls (mean log2FC = 0.294; Chaparral AD proteomics), detected across a single subcellular fraction in TMT-labeled proteomics. The elevated expression may reflect compensatory activation of protein synthesis and ER-associated quality control pathways in response to proteostatic stress characteristic of AD neurodegeneration.