Structure and Function of the Cardiac Stress Response Protein MS1

Myocyte Stress 1 (ms1)/Striated muscle Activator of Rho Signalling (STARS), also known as Actin Binding Rho Activator (ABRA), is a 375 amino acid protein. Its expression increases one hour after the induction of stress in rat hearts through aortic banding. This expression precedes that of early response genes such as c-fos and c-jun. This process has been implicated in the development of left ventricular hypertrophy. ms1/STARS binds to actin, and deletion mutations had shown that residues 234-375 were necessary for actin binding. A mixture of combinatorial domain hunting and rational domain design gave a series of possible domains. Circular dichroism and nuclear magnetic resonance spectroscopy were used to characterise these domains. The first three domains, MSD1 (residues 2-118), MSD2 (40-196) and actin binding domain 1 (ABD1, 193-296) were unfolded, while ABD2 (294-375) was folded. Actin co-sedimentation assays showed that only ABD1 and ABD2 bound to actin. They bound to actin independently. The structure of ABD2 was determined using NMR. Mutation studies, based on the NMR structure and on data about the conservation of positively-charged regions in ms1/STARS homologues, were used to identify the actin binding surface of ABD2. The structure of ABD2 was shown to be a winged helix-turn-helix domain. These domains are often DNA binding domains. When DNA binding was attempted, it was shown that ABD1, ABD2 and the tandem of ABD1 and ABD2 bound to DNA. The identification of the actin binding domain and the discovery of a novel DNA binding ability open up many more possible functions of ms1/STARS.