A TB-RBP and Ter ATPase Complex Accompanies Specific mRNAs from Nuclei through the Nuclear Pores and into Intercellular Bridges in Mouse Male Germ Cells

Abstract

The testis brain RNA-binding protein (TB-RBP) functions as an RNA-binding protein in brain and testis, binding to conserved sequence elements present in specific mRNAs, such as protamine 1 and 2. We show here by RNA gel shift assays, immunoprecipitation, and by a novel in situ hybridization immunohistochemical technique that TB-RBP binds to AKAP4 mRNA in male mouse germ cells. AKAP4 is a component of the fibrous sheath and functions as a scaffolding protein in the sperm flagellum. AKAP4 is encoded by an X-linked gene, is expressed solely in postmeiotic (haploid) male germ cells, and is an essential protein in all spermatozoa, requiring its transport between spermatids as a protein or mRNA. AKAP4 mRNA forms a complex with TB-RBP and the Ter ATPase in nuclei and remains associated with these proteins as it exits nuclei into the cytoplasm and as it passes through intercellular bridges between spermatids. A similar mRNA-TB-RBP-Ter ATPase association is seen for protamine 2 mRNA, which is stored in the cytoplasm of postmeiotic germ cells about 7 days before translation. In contrast, no association is seen with PGK-2 mRNA which is initially transcribed early in meiosis with increased transcription in postmeiotic male germ cells. Although PGK-2 mRNA is subject to translational control, it lacks TB-RBP-binding sequences in its mRNA. The AKAP4 or protamine 2 mRNA-protein complexes dissociate in late-stage male germ cells when the mRNAs are translated. We propose that TB-RBP and the Ter ATPase are part of a complex that accompanies specific mRNAs in haploid mouse male germ cells in intracellular and intercellular movement. The temporal relationship of TB-RBP binding and mRNA inactivation in conjunction with the subsequent dissociation of the mRNA-protein complex at the time of mRNA translation suggests a role in translational suppression and/or mRNA stabilization.

TEX14 is essential for intercellular bridges and fertility in male mice

“…After meiosis, cytoplasmic sharing may be necessary for haploid germ cells to remain phenotypically diploid. Supporting this idea, mRNA (23,24) and organelles (10) have been observed to move between haploid spermatids. Because premeiotic germ cells do not need to compensate for genetic imbalance, another possibility is that intercellular bridges permit cytoplasmic sharing of essential signals for the synchronous cell divisions seen in longitudinal segments of seminiferous tubules (6,8,13) or for ''critical stages,'' such as coordinating the entry into meiosis (25,26).…”

Behavioral and Neurochemical Alterations in Mice Lacking the RNA-Binding Protein Translin

“…Like FMRP, translin shows highest expression in the testis and brain (Han et al, 1995b;Hergersberg et al, 1995;Gu et al, 1998;Finkenstadt et al, 2001). In the testis, translin binds translationally repressed mRNAs containing cis-acting motifs termed Y and H elements and accompanies these transcripts along intercellular bridges between spermatids (Morales et al, 2002). In the brain, translin binds CaMKII␣ and the untranslated RNA BC1 (Kobayashi et al, 1998;Severt et al, 1999;Wu and Hecht, 2000), dendritically localized RNAs that also associate with FMRP (Zalfa et al, 2003).…”

Translin associated protein X is essential for cellular proliferation

“…Translin and its mouse ortholog, Testis brain RNA-binding protein (TB-RBP), are highly conserved and ubiquitously expressed DNA-and RNA-binding proteins with proposed functions in mitotic cell division [1,2], chromosomal translocations [3][4][5], stabilization, transport, and translational suppression of specific mRNAs [6][7][8]. Mice lacking TB-RBP are viable and when heterozygotes were bred, a normal Mendelian distribution was obtained [2].…”