Identification of protein interactors of SPAG1 in human airway epithelia

Dysfunctional motile cilia result in primary ciliary dyskinesia (PCD), a genetically heterogeneous disorder characterized by chronic oto-sino-pulmonary disease, infertility, and laterality defects. Mutations in dynein axonemal assembly factors (DNAAF), that facilitate the assembly of dynein arms in the cytoplasm before transport into the cilium, cause PCD. Sperm-associated antigen 1 (SPAG1) is a DNAAF; however, SPAG1’s precise role in dynein assembly is unknown. Here, we identify a novel 60 kDa SPAG1 isoform that can partially compensate for the absence of full-length SPAG1 to assemble normal outer dynein arms, leading to phenotypic variability in PCD subjects with SPAG1 mutations. Our data shows that SPAG1 interacts with DNAAF, dynein chains, and components of the R2TP complex, and SPAG1 is necessary for axonemal dynein arm assembly by facilitating the folding and/or binding of the dynein heavy chains to the intermediate chain complex.