Mili Interacts with Tudor Domain-Containing Protein 1 in Regulating Spermatogenesis

Abstract

SUMMARY Piwi proteins play essential roles in germline development, stem cell self-renewal, epigenetic regulation, and transposon silencing, and are implicated in oncogenesis [1-7]. Piwi proteins bind to a complex class of small non-coding 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 using co-immunoprecipitation and mass spectrometry. Here we report that Mili specifically interacts with Tudor Domain Containing Protein 1 (Tdrd1; a.k.a. Mouse Tudor Repeat 1, Mtr-1), 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 first two Tudor domains. In addition, Mili positively regulates Tdrd1 expression at the mRNA level. 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 critical for spermatogenesis.