Arginine/serine-rich domains of the su(w^a) and tra RNA processing regulators target proteins to a subnuclear compartment implicated in splicing

Two unrelated pre-mRNA splicing regulators - suppressor-of-white-apricot (su(w^a)) and transformer (tra)-contain distinctive, ∼120 amino acid arginine/serine (RS)-rich domains. Deletion of the su(w^a) RS domain eliminates function. Replacement with the tra RS domain restores su(w^a) function to nearly wild-type levels. Replacement with a 10 amino acid simple nuclear entry signal allows partial, inefficient function. Thus, the su(w^a) RS domain apparently serves a generic function(s) subsuming nuclear entry. Moreover, immunocytochemical studies demonstrate that both RS domains specifically direct localization of a fused reporter protein to a punctate subnuclear compartment shown previously to be enriched in several constitutive splicing components. We propose that RS domains are a new class of targeting signals directing concentration of proteins in a subnuclear compartment implicated in splicing metabolism.