HAX1 controls mitochondrial proteostasis

The relevance of mitochondrial proteostasis for the differentiation and function of hematopoietic and immune cells is unknown. Dissecting the HCLS1-associated protein X-1 (HAX1) –related protein network, we define a functional CLPB-HAX1-PRKD2-HSP27 axis with critical importance for the differentiation of neutrophil granulocytes and thus elucidate molecular mechanisms underlying congenital neutropenia in patients with mutations in HAX1 and CLPB. As shown by SILAC proteomics, CLPB and HAX1 control the balance of protein synthesis and persistence in mitochondria, leading to dysfunction of the respiratory complex 1. Impaired mitochondrial proteostasis is associated with decreased abundance of the serine-threonine kinase PRKD2 and Ser82-phosphorylated HSP27. Cellular defects in HAX1-/- cells can be functionally reconstituted by HSP27. Thus, mitochondrial proteostasis emerges as a critical mechanism governing the differentiation and function of neutrophil granulocytes.