A cytoplasmic variant of the KH-type splicing regulatory protein serves as a decay-promoting factor for phosphoglycerate kinase 2 mRNA in murine male germ cells

Phosphoglycerate kinase 2 (PGK2) is a germ cellspecific protein whose mRNA is translationally regulated in the mammalian testis. Using RNA affinity chromatography with the 3′-untranslated region (UTR) of Pgk2 mRNA and adult testis extracts, several associated proteins including a novel isoform of the AU-rich element RNA-binding protein and KHtype splicing regulatory protein (KSRP) were identified. KSRP, a protein of ∼75 kDa, is widely expressed in somatic and germ cells where it is primarily nuclear. In addition to the ∼75-kDa KSRP, a ∼52-kD KSRP, t-KSRP, is present in the cytoplasm of a subpopulation of germ cells. t-KSRP binds directly to a 93-nt sequence (designated the F1 region) of the 3′-UTR of the Pgk2 mRNA and destabilizes Pgk2 mRNA constructs in testis extracts and in transfected cells. We conclude that this testicular variant of the multifunctional nucleic acid-binding protein, KSRP, serves as a decay-promoting factor for Pgk2 mRNA in male germ cells.