Extended disordered regions of ribosome-associated NAC proteins paralogs belong only to the germline in Drosophila melanogaster

The nascent polypeptide-associated complex (NAC) consisting of α- and β-subunits is an essential ribosome-associated protein conserved in eukaryotes. NAC is a ubiquitously expressed co-translational regulator of nascent protein folding and sorting providing for homeostasis of cellular proteins. Here we report on discovering the germline-specific NACαβ paralogs (gNACs), whose β-subunits, non-distinguishable by ordinary immunodetection, are encoded by five highly homologous gene copies, while the α-subunit is encoded by a single αNAC gene. The gNAC expression is detected in the primordial embryonic and adult gonads via immunostaining. The germline-specific α and β subunits differ from the ubiquitously expressed paralogs by the extended intrinsically disordered regions (IDRs) acquired at the N- and C-termini of the coding regions, predicted to be phosphorylated. The presence of distinct phosphorylated isoforms of gNAC-β subunits is confirmed by comparing of their profiles by 2D-isoeletrofocusing resolution before and after phosphatase treatment of testis ribosomes. We revealed that the predicted S/T sites of phosphorylation in the individual orthologous IDRs of gNAC-β sequences of Drosophila species are positionally conserved despite these disordered regions are drastically different. We propose the IDR-dependent molecular crowding and specific coordination of NAC and other proteostasis regulatory factors at the ribosomes of germinal cells. Our findings imply that there may be a functional crosstalk between the germinal and ubiquitous α- and β-subunits based on assessing their depletion effects on the fly viability and gonad development.