Dynamic in vivo interactions among Myc network members

Members of the Myc oncoprotein network (c-Myc, Max, and Mad) play important roles in proliferation, differentiation, and apoptosis. We expressed chime ric green fluorescent protein (GF-P) fusions of c-Myc,. Max, and three Mad proteins in fibroblasts.. Individually, c-Myc and Mad proteins localized in subnuclear speckles, whereas Max assumed a homogeneous nuclear pattern. Th ese distributions were co-dominant and dynamic, however, as each protein as sumed the pattern of its heterodimeric partner when the latter was co-expre ssed at a higher level. Deletion mapping of two Mad members, Mad1 and Mxi1, demonstrated that the domains responsible for nuclear localization and spe ckling are separable. A non-speckling Mxi1 mutant was also less effective a s a transcriptional repressor than wild-type Mxi1. c-Myc nuclear speckles w ere distinct from SC-35 domains involved in mRNA processing. However, in th e presence of co-expressed Max, c-Myc, but not Mad, co-localized to a subse t of SC-35 loci. These results show that Myc network proteins comprise dyna mic subnuclear structures and behave co-dominantly when co-expressed with t heir normal heterodimerization partners. In addition, c-Myc-Max heterodimer s, but not Max-Mad heterodimers, localize to foci actively engaged in pre-m RNA transcription/processing. These findings suggest novel means by which M yc network members promote transcriptional activation or repression.