Mili interacts with tudor domain-containing protein 1 in regulating spermatogenesis

Piwi proteins are essential for germline development, stem cell self-renewal, epigenetic regulation, and transposon silencing [1-7]. They bind to a complex class of small noncoding RNAs called Piwi-interacting RNAs (piRNAs) [8]. Mammalian Piwi proteins such as Mili are localized in the cytoplasm of spermatogenic cells, where they are associated with a germline-specific organelle called the nuage or its derivative, the chromatoid body, as well as with polysomes [9]. To investigate the molecular mechanisms mediated by Mili, we searched for Mili-interacting proteins. Here, we report that Mili specifically interacts with Tudor domain-containing protein 1 (Tdrd1), a germline protein that contains multiple Tudor domains [10, 11]. This RNA-independent interaction is mediated through the N-terminal domain of Mili and the N-terminal region of Tdrd1 containing the myeloid Nervy DEAF-1 (MYND) domain and the first two Tudor domains. In addition, Mili positively regulates the expression of the Tdrd1 mRNA. Furthermore, Mili and Tdrd1 mutants share similar spermatogenic defects. However, Tdrd1, unlike Mili, is not required for piRNA biogenesis. Our results suggest that Mili interacts with Tdrd1 in the nuage and chromatoid body. This interaction does not contribute to piRNA biogenesis but represents a regulatory mechanism that is critical for spermatogenesis.