Nitric oxide modulates vascular smooth muscle cell (SMC) cytoskeletal kinetics and phenotype, in part, by stimulating cGMP-dependent protein kinase I (PKGI). To identify molecular targets of PKGI, an interaction trap screen in yeast was performed using a cDNA encoding the catalytic region of PKGI and a human lung cDNA library. We identified a cDNA that encodes a putative PKGI-interactor that is a novel variant of TRIM39, a member of the really interesting new gene (RING) finger family of proteins. Although this TRIM39 variant encodes the NH₂-terminal RING finger (RF), B-box, and coiled-coil (RBBC) domains of TRIM39, instead of a complete COOH-terminal B30.2 domain, this TRIM39 isoform contains the COOH-terminal portion of Rpp21, a component of RNase P. RT-PCR demonstrated that the TRIM39 variant, which we refer to as TRIM39R, is transcribed in the human fetal lung and in rat pulmonary artery SMC. Indirect immunofluorescence using an antibody generated against the conserved domains of TRIM39 and TRIM39R revealed the proteins in speckled intranuclear structures in human acute monocytic leukemia (THP-1) and human epidermal carcinoma line (HEp-2) cells. PKGI phosphorylated a typical PKGI/PKA phosphorylation domain in a conserved region of TRIM39 and TRIM39R. Additional studies demonstrated that PKGI interacts with both isoforms of TRIM39 in yeast cells and phosphorylates both isoforms of TRIM39 in human cell lines. Although PKGI has been observed to interact with proteins that regulate cytoskeletal function and gene expression, this investigation shows for the first time that PKGI interacts with tripartite motif (TRIM) proteins, which, through diverse molecular pathways, are often observed to regulate important aspects of cellular homeostasis.