The extended disordered sequences in ribosome-associated germline-specific NAC proteins are their feature compared to the ubiquitously expressed paralog

The nascent polypeptide-associated complex (NAC) consisting of α- and β-subunits is an essential conserved ribosome-associated protein in eukaryotes. NAC is considered as a ubiquitously expressed co-translational regulator of nascent protein folding and sorting providing homeostasis of cellular proteins. Here we discovered the germline-specific NACαβ paralogs (gNACs), whose β-subunits, non-distinguishable by ordinary immunodetection, being encoded by five highly homologous gene copies while α-subunit is encoded by the single αNAC gene. Immunostaining detects the gNAC expression in the primordial embryonic and adult gonads. The germline specific α and β subunits differ from the ubiquitously expressed paralogs by the acquisition of extended intrinsically disordered regions (IDRs) at the N- and C-ends of coding regions, respectively, which were predicted to be phosphorylated. The presence of distinct phosphorylated isoforms of gNAC-β subunits is confirmed by comparison of their profiles by 2D-isoeletrophocusing resolution before and after phosphatase treatment of testis ribosomes. We propose IDR-dependent molecular crowding and specific coordination of the NAC and other proteostasis regulatory factors in the ribosomes of germinal cells. A possibility of the functional crosstalk’s between the germinal and ubiquitous α- and β- subunits is revealed by assessing their depletion effects on the fly viability and gonad development.