Abstract
During spermatogenesis, mRNA localization and translation are believed to be regulated in a stage-specific manner. We report here that the Protamine2 (Prm2) mRNA transits through chromatoid bodies of round spermatids and localizes to cytosol of elongating spermatids for translation. The transacting factor CBF-A, also termed Hnrnpab, contributes to temporal regulation of Prm2 translation. We found that CBF-A co-localizes with the Prm2 mRNA during spermatogenesis, directly binding to the A2RE/RTS element in the 3′ UTR. Although both p37 and p42 CBF-A isoforms interacted with RTS, they associated with translationally repressed and de-repressed Prm2 mRNA, respectively. Only p42 was found to interact with the 5′cap complex, and to co-sediment with the Prm2 mRNA in polysomes. In CBF-A knockout mice, expression of protamine 2 (PRM2) was reduced and the Prm2 mRNA was prematurely translated in a subset of elongating spermatids. Moreover, a high percentage of sperm from the CBF-A knockout mouse showed abnormal DNA morphology. We suggest that CBF-A plays an important role in spermatogenesis by regulating stage-specific translation of testicular mRNAs.
Highlights
In eukaryotic cells, nascent precursor-mRNAs are cotranscriptionally assembled into ribonucleoprotein particles (RNP)
We discovered that the heterogeneous nuclear ribonucleoprotein CArG box binding factor A (CBF-A), interacts with a conserved sequence, the RNA trafficking sequence (RTS), located in the untranslated region of transported mRNAs
We show that by interacting with the RTS of the Protamine 2 mRNA both CBF-A isoforms contribute to regulate the transcript at the translational level
Summary
Nascent precursor (pre)-mRNAs are cotranscriptionally assembled into ribonucleoprotein particles (RNP). RTS recognition by hnRNP A2 has been correlated with MBP mRNA trafficking towards myelin-forming processes and with stimulation of cap-dependent translation [9,10]. Recognition of the MBP mRNA RTS by CBF-A is important for MBP mRNA localization to the myelin compartment [11], which altogether suggests that RNA trafficking mechanisms are likely to be modulated by multiple transacting factors. CBF-A binding to RTS-like sequences of certain dendritic mRNAs was found to be a requirement for activity-dependent transport to neuronal synapses [12]. How these mechanisms work and whether the two known CBF-A splice variants p42 (Hnrnpab1) and p37 (Hnrnpab2) synergize is not known [13,14]. The above observations and similar findings in Xenopus laevis [15,16,17] suggest that CBF-A plays a conserved function which can sort transcripts that are competent for cytoplasmic transport and local translation at specific subcellular compartments
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