Promotes dimerization of NF-kappa-B subunits and regulates NF-kappa-B transcription factor activity (By similarity). Plays a role in the regulation of the growth of actin filaments. Inhibits the activity of the F-actin-capping protein complex formed by the CAPZA1 and CAPZB heterodimer. Promotes growth of cardiomyocytes, but not cardiomyocyte proliferation. Promotes cardiac muscle hypertrophy. Interacts with RELA (By similarity). Interacts with the heterodimer formed by CAPZA1 and CAPZB. Ubiquitous. Up-regulated in heart left ventricle of patients with severe coronary artery disease and history of myocardial ischemia. Up-regulated in heart left ventricle of patients with dilated cardiomyopathy. This protein is produced by a bicistronic gene which also produces the MPD6 protein from a non-overlapping reading frame. MPD6 belongs to a group of cryptic antigens without conventional genomic structure. It is encoded by a cryptic open reading frame located in the 3'-untranslated region of MTPN. Belongs to the myotrophin family.