cGMP kinase I (cGKI) signaling modulates multiple physiological processes including smooth muscle relaxation. The expression of cGKI and its substrate IRAG (Inositol 1,4,5-trisphosphate receptor associated cGMP kinase substrate) was studied. IRAG and cGKI were colocalized in the smooth muscle of aorta and colon. IRAG was present in the thalamus and in most of the myenteric plexus in the absence of cGKI. Coexpression of IRAG and cGKIbeta or cGKIalpha in COS-7 cells revealed that IRAG recruits cGKIbeta but not cGKIalpha to the endoplasmic reticulum. These results suggest that IRAG may be involved in cGKI-dependent and -independent pathways.