The Hippo signalling pathway and its central effector YAP regulate proliferation of cardiomyocytes and growth of the heart. Using genetic models in mice we show that the increased proliferation of embryonal and postnatal cardiomyocytes due to loss of the Hippo-signaling component SAV1 depends on the Myb-MuvB (MMB) complex. Similarly, proliferation of postnatal cardiomyocytes induced by constitutive active YAP requires MMB. Genome studies revealed that YAP and MMB regulate an overlapping set of cell cycle genes in cardiomyocytes. Protein-protein interaction studies in cell lines and with recombinant proteins showed that YAP binds directly to B-MYB, a subunit of MMB, in a manner dependent on the YAP WW domains and a PPXY motif in B-MYB. Disruption of the interaction by overexpression of the YAP binding domain of B-MYB strongly inhibits the proliferation of cardiomyocytes. Our results point to MMB as a critical downstream effector of YAP in the control of cardiomyocyte proliferation.

YAP, a major downstream effector of the Hippo signaling pathway, is an important regulator of cell proliferation. The ability of YAP to regulate G2/M gene expression is dependent on the Myb-MuvB (MMB) complex, consisting of the evolutionary MuvB core complex and the facultative B-MYB subunit, a transcription factor. Here we show that YAP directly binds to B-MYB. Disruption of the YAP/B-MYB interaction by overexpression of the YAP binding domain of B-MYB results in errors in cell division. We also show that YAP and MMB interact in vivo in the developing heart. Genome studies revealed that YAP and MMB regulate an overlapping set of cell cycle genes in cardiomyocytes. Cardiac specific deletion of the LIN9 subunit of MMB prevents the upregulation of cell cycle genes and the increased proliferation of cardiomyocytes lacking the Hippo-signaling component SAV1. Similarly, we find that proliferation of postnatal cardiomyocytes induced by constitutive active YAP depends on MMB. Our findings provide new insights in the YAP induced proliferation of cardiomyocytes through the functional interaction with MMB.