HnRNP G and Tra2β: opposite effects on splicing matched by antagonism in RNA binding

The hnRNP G family comprises three closely related proteins, hnRNP G, RBMY and hnRNP G-T. We showed previously that they interact with splicing activator proteins, particularly hTra2β, and suggested that they were involved in regulating Tra2-dependent splicing. We show here that hnRNP G and hTra2β have opposite effects upon the incorporation of several exons, both being able to act as either an activator or a repressor. HnRNP G acts via a specific sequence to repress the skeletal muscle-specific exon (SK) of human slow skeletal alpha-tropomyosin, TPM3, and stimulates inclusion of the alternative non-muscle exon. The binding of hnRNP G to the exon is antagonized by hTra2β. The two proteins also have opposite effects upon a dystrophin pseudo-exon. This exon is incorporated in a patient to a higher level in heart muscle than skeletal muscle, causing X-linked dilated cardiomyopathy. It is included to a higher level after transfection of a mini-gene into rodent cardiac myoblasts than into skeletal muscle myoblasts. Co-transfection with hnRNP G represses incorporation in cardiac myoblasts, whereas hTra2β increases it in skeletal myoblasts. Both the cell specificity and the protein responses depend upon exon sequences. Since the ratio of hnRNP G to Tra2β mRNA in humans is higher in skeletal muscle than in heart muscle, we propose that the hnRNP G/Tra2β ratio contributes to the cellular splicing preferences and that the higher proportion of hnRNP G in skeletal muscle plays a role in preventing the incorporation of the pseudo-exon and thus in preventing skeletal muscle dystrophy.