The mitochondrial tumor suppressor 1 (Mtus1) gene codes for the angiotensin II type-2 receptor-interacting protein (ATIP). Originally, ATIP was identified as mitochondrial protein exerting anti-proliferative effects but also regarded as a novel interacting partner of the angiotensin II type-2 receptor. However, the physiological role of ATIP in the cardiovascular system is unknown. To address this question, we generated mice with a trapped Mtus1 gene. Expression studies by Northern blot, RT-PCR, Western blot, and ex vivo X-Gal promoter studies revealed a high promoter activity and expression level of ATIP in the heart. Using confocal laser scanning microscopy, a plasma membrane- associated localization of ATIP in cardiac myocytes was detected, which is in sharp contrast to the previously proposed mitochondrial localization. Functionally, Mtus1 knockout mice develop a spontaneous cardiac hypertrophy, demonstrated by an increased heart/body weight ratio of 6.4 vs. 4.6 x 10^-3 of wild-type controls (n=16; p<0.05), enhanced cardiac myocyte diameters 14.88 vs. 11.03 mm in wild-type animals (n=12; p<0.05) and myocardial fibrosis (1.97[asymp]0.09-fold relative collagen content vs. wild-type). Non- invasive blood pressure measurements did not provide significant differences between knockout mice and controls, neither in systolic nor in diastolic pressure (SBP WT/KO 110/107, DBP WT/KO 70/68, n=16). However, a tendency to a lower systolic pressure in elder Mtus1 knockouts was seen, which might reflect the beginning of cardiac dysfunction. We conclude that Mtus1 gene disruption results in spontaneous cardiac hypertrophy in vivo and speculate that impaired angiotensin II type-2 receptor signaling causes this cardiac dysfunction.