SPNR, A MURINE RNA-BINDING PROTEIN THAT IS LOCALIZED TO CYTOPLASMIC MICROTUBULES

Previous studies in transgenic mice have established the importance of the 3' untranslated region (UTR) of the spermatid-specific protamine- 1 (Prm-1) mRNA in its translational control during male germ cell deve lopment. To clone genes that mediate the translational repression or a ctivation of the Prm-1 mRNA, we screened cDNA expression libraries mad e with RNA from pachytene spermatocytes and round spermatids, with an RNA probe corresponding to the 3' UTR of Prm-1. We obtained six indepe ndent clones that encode Spnr, a spermatid perinuclear RNA-binding pro tein. Spnr is a 71-kD protein that contains two previously described R NA binding domains. The Spnr mRNA is expressed at high levels in the t estis, ovary, and brain, and is present in multiple forms in those tis sues. Immunolocalization of the Spnr protein within the testis shows t hat it is expressed exclusively in postmeiotic germ cells and that it is localized to the manchette, a spermatid-specific microtubular array . Although the Spnr protein is expressed too late to be directly invol ved in the translational repression of Prm-1 specifically, we suggest that the Spnr protein may be involved in other aspects of spermatid RN A metabolism, such as RNA transport or translational activation.