Microsatellite-encoded domain in rodent Sry functions as a genetic capacitor to enable the rapid evolution of biological novelty

Abstract

The male program of therian mammals is determined by Sry, a transcription factor encoded by the Y chromosome. Specific DNA binding is mediated by a high mobility group (HMG) box. Expression of Sry in the gonadal ridge activates a Sox9-dependent gene regulatory network leading to testis formation. A subset of Sry alleles in superfamily Muroidea (order Rodentia) is remarkable for insertion of an unstable DNA microsatellite, most commonly encoding (as in mice) a CAG repeat-associated glutamine-rich domain. We provide evidence, based on an embryonic pre-Sertoli cell line, that this domain functions at a threshold length as a genetic capacitor to facilitate accumulation of variation elsewhere in the protein, including the HMG box. The glutamine-rich domain compensates for otherwise deleterious substitutions in the box and absence of nonbox phosphorylation sites to ensure occupancy of DNA target sites. Such compensation enables activation of a male transcriptional program despite perturbations to the box. Whereas human SRY requires nucleocytoplasmic shuttling and coupled phosphorylation, mouse Sry contains a defective nuclear export signal analogous to a variant human SRY associated with inherited sex reversal. We propose that the rodent glutamine-rich domain has (i) fostered accumulation of cryptic intragenic variation and (ii) enabled unmasking of such variation due to DNA replicative slippage. This model highlights genomic contingency as a source of protein novelty at the edge of developmental ambiguity and may underlie emergence of nonSry-dependent sex determination in the radiation of Muroidea.nucleocytoplasmic trafficking | protein-DNA recognition | sexual dimorphism | transcriptional activation | triplet expansion

Inherited human sex reversal due to impaired nucleocytoplasmic trafficking of SRY defines a male transcriptional threshold

“…6B, Bottom. Epitope-tagged hSRY variants containing PKA-site substitutions RRAAAFL ("phospho-dead") or RRDDDFL ("phospho-mimic") were not detected as further described in a recent study (22); these findings indicate that, in CH34 cells, this is the only site of serine phosphorylation (green box in Fig. 6C).…”

“…Their HMG boxes in each case contain consensus NES sequences (IxxxLxxxxxML). The divergent boxes of rat and mouse Sry by contrast contain an unfavorable NES substitution (M→S), shown in a recent study to impair nuclear export in association with gain of a microsatellite-associated transcriptional activation domain (22). Whereas I90M (HMG position 35) occurs within α-helix 2 and perturbs packing of the major wing, the rodent substitution occurs in a loop following this α-helix and so may be better tolerated.…”

“…The similar transcriptional thresholds of murine and human SRY [despite their biochemical divergence; see a recent study (22)] suggests that its thin thread of function (29) is the product of selection. The shared tenuousness of the switch poses an intriguing problem given the general robustness of developmental processes (61).…”

“…Although NCS-dependent transcriptional regulation is likely to be broadly conserved among Sox genes (SI Appendix, Table S2) (19,20), its importance to the function of SRY has been obscure in studies of mouse models (21). In a recent study (22) we demonstrated that murine Sry fails to undergo NCS as a consequence of an inactive nuclear export signal (NES) motif. Whereas the HMG box of hSRY contains an active NES (IxxxLxxxxxML), microsatellite-associated rodent variants contain the variant sequence IxxxLxxxxxSL (22).…”

“…In a recent study (22) we demonstrated that murine Sry fails to undergo NCS as a consequence of an inactive nuclear export signal (NES) motif. Whereas the HMG box of hSRY contains an active NES (IxxxLxxxxxML), microsatellite-associated rodent variants contain the variant sequence IxxxLxxxxxSL (22). Such variation is associated with clade-specific insertion of a DNA microsatellite in the divergent evolution of the Y chromosome in superfamily Muroidea (order Rodentia) (23).…”